A physics-based material processing simulation is approached to reveal the distortion law of aerospace monolithic components due to milling. To improve the efficiency of numerical calculation, a simplified cutting layer model is presented. The cutting force and temperature model are established to gain cutting loads. The cutting loads are applied to the mesh model of the part. The milling process of spar is simulated. Key factors influencing distortion such as initial residual stress, cutting loads, cutting sequence and tool path are considered. The total distortions of the spar are estimated. Machining experiment is carried out, and the distortion tendency of the machined part agrees well with the simulation. The results show the validity of the presented approach.