The size error is easy to produce in the process of slender shaft. In order to ensure the machining accuracy of slender shaft, the small machining method is usually adopted, which makes the machining efficiency low. Aiming at the problem of low efficiency of the low stiffness slender shaft, the mirror support of the cutting fluid was designed to increase the rigidity of the slender shaft. This method could increase the cutting amount. The objective function of the processing time was established by the method of mirror support of the cutting fluid. In the process of solving the model, constraint conditions including the condition of the actual device and maximum size error were established. And cutting parameters optimization was solved, pursuing the goal of minimum machining time under mirror support of cutting fluid. Finally, the experiment verifies the availability of this model. The results show this method can improve the processing efficiency.