The invention discloses a heat treatment process of hot work die steel. The hot work die steel comprises the following chemical components including, by weight, 0.4%-0.85% of C, 2.6%-3.3% of Cr, 4.1%-4.2% of Mo, 5.2%-5.5% of W, 0.65%-0.95% of V, 0.3%-0.35% of Mn, 0.7%-1.3% of Ni, 0.36%-0.52% of Cs, smaller than or equal to 0.01% of S, smaller than or equal to 0.005% of P and the balance Fe and inevitable impurities. The heat treatment process comprises a first-order temperature rising and returning step, a second-order temperature rising and returning step, a third-order temperature rising andheat preservation step, a cooling step, a copious cooling step and a tempering step. According to the heat treatment process, the hot work die steel with the optimized components is adopted for multi-stage temperature rising and returning treatment, the effect of uniform thermal stress diffusion is achieved, pressure maintaining cooling, copious cooling and tempering are combined, the impact toughness and the high-temperature stability can be considered while the high-hardness requirement of the die steel is met, and the comprehensive mechanical property is greatly improved.