The thin-walled cavity workpiece with insufficient rigid property is liable to deform during the machining process and the request of accuracy is very strict. The paper takes typical aeronautic aluminum-alloy for example, fixture is an important consideration in the operation. To reveal the influences of locating points, clamping sequence and loading ways on the distortion of thin-walled cavity part, finite element models were established to simulate the clamping operation. The result shows the preferable scheme is that the distance of the clamping locations are far each other, clamping forces are firstly applied on the surface with high rigid and all clamping forces are applied in many steps. The scheme can effectively control the deformation of clamp ,and furthermore improve the machining accuracy.