Abstract: When employing the currently widely used three-parameter equation to fit 5 groups ofa-Ntest data of crack propagation, it is found that the fitting precision of the equation for the whole course test data of crack propagation is not high. Especially whenNapproaches the fracture life, the fitting error is getting very large. Therefore, the authors put forward a four-parameter equation, in which the unknown parameters can be obtained by evaluating the extreme value points of poly-function. It is indicated that when fitting the whole coursea-Ntest data of crack propagation, the fitting precision of the four-parameter equation is higher than that of three-parameter equation.