The stability of reheat steam system of coalfired unit affects the safety and economy of the system. Based on the operation data of 6 units (A to F) of the same group corporation,the stability benchmarking evaluation of reheat steam system was carried out in this paper. The big data was cleaned through the 3σ criteria,and the stable working conditions were classified by using the sliding pane method. Based on the process capability index Cpk and coefficient of variance CV,the stability of reheat steam temperature and desuperheating water regulating valve opening of reheater was scored respectively,and the weight distribution of parameters was calculated by analytic hierarchy processtechnique for order preference by similarity to ideal solution (AHPTOPSIS) to complete the comprehensive benchmarking evaluation of different units under different working conditions. The results show that in units A to F,the corresponding value of reheat steam temperature of unit E is the largest,Cpk is 1.75,and its stability is the best; the corresponding value of desuperheating water regulating valve opening of reheater of unit F is the smallest,CV is 0.56,and its stability is the best;the stability of reheat steam temperature and desuperheating water regulating valve opening of reheater of unit A is the worst.The left and right steam temperature deviation of unit A and unit C and the difference of desuperheating water regulating valve opening of reheater are obviously too large,which is due to its fourcorner tangential combustion;the reheat steam system of 300 MW unit E has the best stability,and the reheat steam system of 1 000 MW unit has poor stability.