Based on the irreversible phenomenon of the Cartesian diver，it analyzes the sinking process of the Cartesian diver and the reasons why the Cartesian diver is irreversible by using Archimedes’principle and Boyle’s law．The factors of critical depth are obtained by establishing the critical depth model of the Cartesian diver．Then the theoretical function curve of the critical depth changing are made by using the control variable method to explore the effect of each factor on the critical depth．Comparing the experimental data with the critical depth model，the average relative error of the two is less than 4．13%．It provides theoretical support and  experimental methods for the research on the Cartesian diver．