A three-step heat treatment process for 42CrMo crankshaft forged steel for diesel vehicles comprises the following steps of normalizing treatment, tempering treatment, surface high-frequency quenchingand the like. Firstly, a primary normalizing heat treatment process is added as a preheating process before the 42CrMo crankshaft forged steel for the vehicles only needs a tempering treatment process; the normalizing heat treatment process is selected as 880 DEG C*1h air cooling, and when the temperature is raised to 550 DEG C, the temperature is kept for 0.5h; a crankshaft subjected to normalizing heat treatment is subjected to tempering treatment including quenching and high-temperature tempering treatment, special quenching liquid is selected as a quenching medium instead of water coolingand oil cooling which are commonly used, the specific heat preservation time is prolonged to 5 hours during high-temperature tempering and then air cooling is carried out, and finally high-frequencyinduction quenching treatment is carried out on a journal and the surface part of a fillet, and specific process parameters and performance index requirements are given out.