Thin-walled aero-engine casings have complex flow channel structure. In order to complete the milling machining and obtain higher machining accuracy, both ends need to be positioned and clamped respectively. The clamping technology of the casings was studied based on the clamping requirements. The finite element method was used to perform harmonic response analysis, and the clamping mode and pressing position were selected according to the deformation of the casings. A special fixture was designed according to the simulation results, and the compound clamping method of center hole positioning, bottom edge and upper-lower auxiliary clamping was adopted to effectively limit the six degrees of freedom of the casings. The calculation results show that the positioning error of the machine tool-fixture-casings meets the accuracy requirements, and the actual machining results show that the casings flow channel after repeated clamping and processing meets the machining accuracy and surface roughness requirements.