Owing to such features as lightweight, integrity, the aeroengine casing evolves into complex structure, large dimension, thin walls. Associated with increasing requirement for accuracy, casing is prone to severe distortion after machining. This research investigated the mechanism of casing distortion and proposed a strategy of deformation control based on the allowance division. The surface residual stress of TC4 forging casing blank was measured by the ESPI (Electronic Speckle Pattern Interferometry) technologies. Then, the inner residual stress distribution of TC4 casing was deduced from the finite element simulation. Machining process of TC4 casing was modelled and simulated. Two simulation strategies were proposed and optimized by comparing the deformation. Finally, the simulation result was verified by experiment for machining a casing under optimized plan in the workshop.