A rapid optimization algorithm to determine the selective value area of input mirror reflectivity and output mirror reflectivity for the optimum of high power fiber lasers is proposed. Under slope efficiency optimization selection criterion,taking Yb3+-doped fiber lasers as examples,the numerical simulation results indicate that computing time using the rapid optimization algorithm of reflectivity in high power fiber lasers is less than 8% of time using traditional algorithm,ladder errors are well avoided and improvement of precision is obtained. The numerical simulation results also indicate that under the conditions,the selective area of mirror reflectivity of fiber lasers with forward pumping is largest,the selective area of mirror reflectivity of fiber lasers with bidirectional pumping takes second place and the selective area of mirror reflectivity of fiber lasers with backward pumping is smallest.