Aiming at the cavitation and erosion problems existing in the valve spool and valve sleeve of the water relief valve, the spool structure with high pressure drainage hole and multistage series valve port was designed. The design principle of key matching size of valve spool and valve sleeve was introduced. The mechanism of micro-jet and erosion was analyzed by Fluent, the anti-cavitation principle of multistage series valve spool was deduced. The mathematical model of the water relief valve was established according to the actual structure of the improved spool. In order to verify the anti-cavitation and anti-erosion performances of the improved valve spool， two kinds of valve spool fluid simulation models were established, the influence mechanism of high pressure drainage port and multistage series valve port on cavitation and erosion was explored. The simulation results show that: under the same pressure grade, the improved spool has lower beam flow rate and lighter cavitation phenomenon, which is conducive to the long-term stable operation of water relief valve.