I have got a method, but I am not sure if it is the best choice, if anyone knows, please share and discuss.
The drag force is a local variable in MFIX.
1st, define a global variable
2nd, assign local variable to global variable
3rd,VTK output for user-defined variables of particles
Assign the drag force to the user-defined variable
4th, you can get the values in the vtk file
Another method,
You can just write it out if you are not intended to show the distribution of drag force
Write codes in the drag-force file and write it out by global id of particles