Abstract: Aiming at the problem of uncertain excessive short-circuit current in power system, a flexible tripping strategy was proposed in this paper. Firstly, a fault current prediction algorithm based on the recursive least square principle was used to obtain the preliminary short-circuit current prediction result before the operation of ultra-highspeed protection, so as to determine whether there is a risk of exceeding the interrupting capacity when the protection tripping signal directly starts the circuit breaker’s opening. After monitoring that the protection operation sign becomes "tripping", the prediction of short-circuit current was refreshed, by which the result of the first judgment was checked. Meanwhile, the feasible opening interval was searched within the given time, calculating the unblocking time of the protection to realize the safe-opening of the over-limit circuit breaker. Finally, based on PSCAD/MATLAB platform, the excessive short-circuit current scenes of 500 kV transmission system were simulated. The simulation results show that the proposed flexible tripping strategy can effectively avoid the risk of forced-opening of the over-limit circuit breaker even in the extremely severe fault scene where the short-circuit current exceeds 50% of the interrupting capacity.