Shortcomings in frequency adjustment of UHV locking strategy in provincial power grid AGC system are analyzed. A novel composite locking strategy of provincial power grid AGC system is proposed based on UHV power deviation and interconnected power system frequency offset. If frequency recovery contradicts UHV power deviation adjustment, composite locking strategy judges whether power frequency exceeds the limit. If power frequency is over limited, AGC in provincial power grid acts normally to recover the frequency preferentially. Otherwise AGC system is locked until UHV power deviation decreases to allowable range. Simulation on Matlab/Simulink platform shows the composite locking strategy can restore power grid frequency much faster than UHV locking strategy, does not increase UHV power deviation excessively, and can consider both the power grid freguency regulation and UHV frequency reguletion.