how to find all documents belong to certain folder?

As a!deleteDocument can only delete a single document, you will need to create a process to delete multiple documents.

Documents in Appian are referenced by their ID either in a variable of type Integer or of type Document.

a!localVariables(
  local!oldfile: a!refreshVariable(folder(31338, "documentChildren")),
  local!file: a!refreshVariable(folder(31338, "documentChildren")),
  local!deletefile,
  local!oldfilelist,
  local!testfile:rule!testfilecasttype(),
  {
    a!fileUploadField(
      label: "File Upload",
      labelPosition: "ABOVE",
      target: cons!fileUploadConstant,
      maxSelections: 10,
      value: local!file,
      saveInto: {
        a!save(
          target: local!oldfile,
          value: folder(31338, "documentChildren")
        ),
        /*if(length(local!oldfile)=0,{},a!deleteDocument(document:local!oldfile)),
        */
        a!save(target: local!file, value: save!value),
        a!save(target:local!oldfilelist,value:a!forEach(items:local!oldfile,
        expression:tostring(fv!item)
        /*extract(left(fv!item,20),":","]"),*/
        )),
        a!save(
          target: local!deletefile,
          value: a!forEach(
            items: local!oldfile,
            expression: if(
              contains(local!file, fv!item) = true(),
              {},
              fv!item
            ),
          )
        ),
        a!save(target:local!testfile.file,value:local!oldfilelist),
        a!save(
          target: ri!file.file,
          value: a!forEach(items: local!file, expression: fv!item, )
        ),
        
      },
      fileNames: { fv!file.name },
      validations: {},
      uploadMethods: "CHOOSE_FILE"
    ),
    a!buttonArrayLayout(
      buttons: {
        a!buttonWidget(
          label: "Submit",
          style: "PRIMARY",
          submit: true(),
          loadingIndicator: true(),
          saveInto:{
            /*if(*/
              /*or(*/
                /*length(a!flatten(local!deletefile)) = 0,*/
                /*len(local!deletefile) = 0*/
              /*),*/
              /*{},*/
              /*a!deleteDocument(document: local!deletefile)*/
            /*),*/
            /*a!writeToMultipleDataStoreEntities(*/
            /*valuesToStore: {*/
            /*a!entityData(entity: cons!testfileconstant,data:local!testfile),*/
            /*}*/
            /*),*/
            a!startProcess(
              processModel: cons!wangtestfileuploadprocessmodel,
              processParameters: {
                deletefile:local!deletefile,
                testfile:local!testfile,
                
              }
            ),
          }
          
        )
      },
      align: "START"
    ),
    a!milestoneField(
      label:"phase",
      labelPosition: "ABOVE",
      steps: cons!wangtestphase,
      links: {},
      active: 2,
    )
  }
)

local!deletefile shows are multiple, but once get into processModel it changes to be single always, though i set property values and others to multiple

Did you set the process variable deletefile to be a multiple in the model?

The first as original one can't set to be multiple, so i create a new one at second set it to multiple but not works

You cannot just add inputs and expect this to work!!!

This node can only delete a single file. To delete multiple you need to execute the node multiple times.

You cannot just add inputs and expect this to work!!!

This node can only delete a single file. To delete multiple you need to execute the node multiple times.

Which one is the thing to set nodes? user input task?

seems it is in category of others of that delete document

I just made it 

I do not understand your question. What would a user input task help?

But I assume that this is what you are looking for: docs.appian.com/.../Process_Model_Recipes.html

where to find the multiple instances activity, it looks like user input task

docs.appian.com/.../Process_Node_and_Smart_Service_Properties.html