Cyclic voltammetry and linear sweep voltammetry are the most commonly used diagnosis methods to estimate the internal statement of fuel cell stack. However, both methods can only be applied in a single fuel cell. There is a lack of suitable in situ diagnosis methods for a multi-burl fuel cell stack. The galvanostatic charge method (GSC) is a very convenient in situ diagnosis method, which can be applied to a multi-burl fuel cell stack to calculate the electrochemical active surface area (ECSA), double-layer capacitance, and hydrogen crossover current. However, there are not enough experiments to analyze the adaption of GSC or apply this method to analyze fuel cell degradation process. In this study, we conducted experiments to validate the accuracy of GSC under different test conditions, and proposed a new correction algorithm to improve the accuracy. Next, this method was applied to analyze the performance degradation process of a four-cell stack. The experimental results showed that the estimated GSC parameters exactly coincide with the standard values. Additionally, for the degradation analysis of a four-cell stack, the GSC results showed that the ECSA reduction of cell four is the direct reason for performance degradation. Moreover, the assumptions of ECSA reduction and carbon corrosion were validated by material experiments. About 5 degrees decrease in the contact angle of the gas diffusion layer (GDL) in the cathode catalyst was observed in the worst cell.