Aiming at forging defects and requirements of material processing performance for large modular forgings, for automobile engine crankshaft forging die, the heat treatment process after forging was formulated and the finite element analysis was carried out in the numerical simulation software Deform-3D. Then, taking five factors such as material preparation time, pre-heating temperature, pre-heating time, normalizing temperature and normalizing time as design variables，and the total heating time and the maximum residual stress as the objective functions, the multi-objective orthogonal optimization design for the first half of heat treatment process after forging was conducted. Furthermore, the test results were processed by gray correlation analysis method, and the optimal design method and process parameters were obtained. The results show that the optimized heat treatment process has greatly shortened the total heating time by 26%, and the residual stress has also been reduced. Thus, while ensuring the processing performance of modular forgings, the production efficiency is improved.