The invention discloses a deformation compensation method for machining arc-shaped parts and relates to the field of numerical control machining. The method includes the following steps that A, deformation numerical values are measured; B, statistics is performed on the deformation values; C, deformation compensation numerical values are calculated; D, rough machining is performed; E, a beam is additionally installed; F, numerical control machining numerical values are set; H, finish machining is performed on outer shapes; I, finish machining is performed on inner shapes; J, the beam is disassembled. Deformation directions and the deformation numerical values are estimated in advance, then in the process of numerical control machining, the deformation quantity is set apart, so that after machining, deformation is inevitably generated, the deformed arc-shaped parts just meet the size requirements of the arc-shaped parts, stress is not generated any more in the assembly process, and therefore the service life of the arc-shaped part can be effectively prolonged. The method is easy and convenient to operate. In addition, parameters are very convenient and rapid to adjust if in need, and the deformation compensation method has very practical value in the aspects of predicating and controlling the arc-shaped parts in the numerical machining process.