The invention discloses a secondary reheating unit reheating steam temperature control method. The secondary reheating unit reheating steam temperature control method comprises a flue gas recirculation fan variable-frequency control method and a flue gas adjusting baffle opening degree control method; and a leading differential feedforward unit for the reheating steam temperature after water spraying and a feedforward unit for load changing are arranged in each control method, and meanwhile an overriding and locking control unit for special circumstances is arranged in each control circuit. According to the secondary reheating unit reheating steam temperature control method, the problems that the primary reheating steam temperature and the secondary reheating steam temperature of an ultra-supercritical secondary reheating unit intercouple with each other and are low in adjusting speed are solved, the total heat absorption capacity of a primary reheater and a secondary reheater is controlled through flue gas recirculation fan variable-frequency controllers separately, the primary reheating steam temperature and the secondary reheating steam temperature are controlled through reheater flue gas adjusting baffles, the control measures such as feedforward, overriding and locking are taken, the control precision and the response speed of the primary reheating steam temperature and the secondary reheating steam temperature of the ultra-supercritical secondary reheating unit are achieved, and meanwhile safe and stable running of the unit is guaranteed.