I want some form field was required(not empty),how can I do?
you can create and customize the properties for the Camunda Modeler with the Element Template extension. With Element Templates it is also possible to set properties as required.
Have a look at Element Templates here.
Which type of forms? Generated Task Forms?
The look at this: https://docs.camunda.org/manual/7.10/user-guide/task-forms/#form-field-validation