Computer aided design of heat treatment for AISI P20 mold steel with good machinability is attempted to proceed by the commercial software package Thermo-Calc (TCP+DICTRA). Through experimental and theoretical analysis of phase transformation during heat treatment, further knowledge of designing proper heat treatment is obtained. Then the machinability of AISI P20+Ni steel under given heat treatment condition is studied and the influencing factors to their machinability are analyzed. It is shown that heat treatment designed by computer simulation of carbide transformation is applicable to AISI P20+Ni steel with good machinability; AISI P20+Ni steel with tempered sorbite treated by quenching & tempering has optimal machinability; normalizing at the temperature of 910°C & tempering can avoid cracking and result in acceptable machinability in small thickness module. Computer aided design of heat treatment for AISI P20 mold steel with good machinability is attempted to proceed by the commercial software package Thermo-Calc (TCP + DICTRA). Through experimental and theoretical analysis of phase transformation during heat treatment, further knowledge of designing proper heat Then the machinability of AISI P20 + Ni steel under given heat treatment condition is studied and the influencing factors to their machinability are analyzed. It is shown that heat treatment designed by computer simulation of carbide transformation is applicable to AISI P20 + Ni steel with good machinability; AISI P20 + Ni steel with tempered sorbite treated by quenching & tempering has optimal machinability; normalizing at the temperature of 910 ° C & tempering can avoid cracking and result in acceptable machinability in small thickness module.