The digital prototype of a pilotoperated piston shutoff valve was established, and the principle of the pilotoperated piston shutoff valve was introduced. The dynamic numerical simulation analysis of spool unloading structure was made with the CFD simulation software FLUENT. The results show that: the percentage of spool occupied bonnet cavity cross section is 15%, the percentage of unloading tank occupied bonnet cavity height is 50%, the percentage of pilot hole occupied dampening chamber section is 7%, the percentage of the damping chamber occupied the spool cross section is 30%, the negative pressure value resulted in pilot lever is maximal. It provides theoretical references for the structure design and optimization of pilotoperated piston shutoff valve or other valves.