The accuracy design is an effective method to improve machining accuracy of machine tools. How to identify key geometric errors accurately and assign weights to each geometric error are the prerequisites for achieving accuracy design. Therefore, a sensitivity analysis method for identifying the key geometric errors is proposed. Firstly, the machining error model of machine tools based on the multi-body system theory is formulated. And the sensitivity analysis model based on the machining error model of machine tools is established. Meanwhile, a new sensitivity index is defined. Then, the 10th, 17th, 22nd, 24th and 37th geometric errors are determined as the key geometrical errors of the machine tool by simulation analysis. Meanwhile, the weight of each geometric error is allocated. Finally, the “S” shape test piece is selected as the research object for experiment verification. The “S” shaped test pieces are machined by compensating the identified key geometric errors and all geometric errors. And the form errors of the “ S” shape test piece are compared. The comparison results show that the difference of the average profile error obtained by two compensation strategies on three detection lines is very small, which is 0. 005, 0. 004, and 0. 006 mm, respectively. Therefore, the correctness of sensitivity analysis method is verified.