The cosine error of the diffraction grating ruling engine is a systematic error caused by the measurement system. This error affects grating performance directly. To reduce the effects of the cosine error on grating performance, we analyze the cause of the cosine error, establish a mathematical model based on the error and the related grating indicators, and propose a method to reduce the error. To validate the proposed method, we present the results of grating ruling experiments performed before and after cosine error correction, which show that the method effectively reduces the cosine error effects.