Dear Cheng,
I’m not sure I know enough about Simulink to comment, but Leonardo.Lopez uploaded his Simulink implementation of the baseline controller for the NREL 5-MW turbine in the Mar 03, 2017 post in the following forum topic: Baseline NREL 5MW controller on simulink - #13 by Jason.Jonkman.
The MIN(MAX(,),) logic simply saturates the integral term between the minimum and maximum pitch limits to ensure a fast response in the transitions between Regions 2 and 3.
Best regards,