The invention discloses a pre-hardening type plastic mold steel plate core part retained austenite decomposition method. A high temperature recrystallization controlled-rolling process is adopted for rolling a steel plate, and the cooling speed of the steel plate on a cooling bed is controlled to range from 1 DEG C/s to 3 DEG C/s; when the steel plate is cooled to the scope being MS+20 DEG C-50 DEG C, the steel plate is hoisted into a slow cooling box to be subject to stack cooling for 48 hours or longer; after the steel plate is discharged from the box, high-temperature tempering is carried out at the temperature ranging from 530 DEG C to 630 DEG C; the steel plate is subject to tempering, furnace discharging and air cooling, when the surface temperature of the steel plate drops to 250 DEG C to 280 DEG C, the steel plate is placed into the slow cooling box to be subject to stack cooling for 12 hours or longer, then air cooling is carried out until the indoor temperature is obtained, and the structure of the steel plate is lower bainite and a small amount of wee dispersed retained austenite structure. By means of the pre-hardening type plastic mold steel plate core part retained austenite decomposition method, the saw cutting difficulty and the saw cutting face protruding problem can be avoided without increasing energy consumption, equipment investment and manufacturing cost. The mold machining performance is improved, and the machining accuracy is guaranteed.