Extruded insulation is a major kind of insulation used in power cable. It is significant to evaluate extruded insulation lifetime suffered from electrical aging, which can improve the reliability of power cable. In this paper, a step-stress test method is used in accelerated life tests and Weibull model is used to statistic analysis for the mean lifetime of several samples under the same voltage value, and then the least square method is used to calculate the parameter of Weibull distribution. This paper uses inverse power function to fit test data and calculate voltage endurance coefficients. Constant-stress test data is also used as the counterpart of that measured by the step-stress test. It is indicated that the test voltages of step-stress test are more easily selected than that of constant tests and the data of the former is less scattered and the result is more reliable.