Abstract: This paper introduces the application principle of PDCA cycle, introduces the meaning of “5G”, “5W1H”, “4M” and “5Why”, and organically combines these four tools with PDCA cycle. Taking the actual quality problem of the rear axle support fracture of a Combine as the carrier, this paper explains in detail how to apply the PDCA cycle method and “5G” tools to effectively obtain information; Use the tool of “5W1H” to accurately describe the problem; The “4M” based tool eliminates the influence of "man", "machine" and "method", and determines that the root cause is "material". Through the inspection of the dimension, chemical composition, metallurgical structure and mechanical properties of the "material" of five groups of samples, it is determined that the unqualified factor is that the chemical composition, metallurgical structure and mechanical properties of the parts can not meet the design requirements; The fundamental influencing factor determined by “5Why” analysis method is the chemical composition of the part material. Then, corresponding rectification measures are formulated, implemented and tracked for the root cause that the chemical composition does not meet the design requirements. Then track and verify the rectification to realize standardized operation. This paper can provide standardized analysis methods for applying PDCA cycle to solve quality problems and find the root causes of defects. At the same time, it can be extended to other fields as a problem-solving process.