In recent years, quality-related fault diagnosis has attracted much attention from academic circles. Several fault diagnosis algorithms based on post-processing have been developed. However, further studies have found that these post-processing methods will gradually lose their function when the magnitude of quality-unrelated faults increases. In addition, post-processing algorithms will generate a lot of calculations in practice. In order to further solve the drawbacks of the above methods, this paper adopts a structure of preprocessing, modeling, and postprocessing. And proposed a modified orthogonal projection to latent structures (MOPLS). Compared with the previous algorithm, this method is more practical for quality-related faults. At the same time, the number of latent variables required by the model is reduced. Therefore, its computational complexity is lower than the previous algorithm. Numerical examples and Tennessee-Eastman (TE) process are used to verify the effectiveness of the method.