Weibull distribution is widely used in component failure distribution fitting. Compared with two-parameter Weibull distribution, three-parameter Weibull distribution does better in the situation that aging characteristics does not exist in protective relays at initial operation. This paper analyzes the failure characteristics of protective relay, and proposes a new fitting method for aging failure data of protective relay based on the least square method. Combined with some real grid data, the failure rate function is calculated. The calculation results shows that, compared with two-parameter, three-parameter Weibull distribution is more in line with the aging failure characteristics of protective relay.