With the constant expansion of power grid, a large number of security constraints have put tremendous challenges to the online economic dispatch. An efficient tool to eliminate the inactive constraints becomes necessary for security constraints economic dispatch (SCED) in large power systems. In this paper, considering the uncertainty of renewable energy integration, a fast and effective method to identify the inactive security constraints was proposed. Firstly, the definition of uncertain inactive security constraints and a necessary and sufficient condition to identify those were presented respectively. Considering the time-consuming identification, we relaxed the feasible region of original SCED model and utilized greedy algorithm to quickly eliminate part of inactive constraints. Then another necessary and sufficient condition was proposed and verified, where the remaining inactive constraints could be identified. Through the simulation of 24-time periods 9-bus system and 96-time periods IEEE 118-bus system, the speed and effectiveness of proposed method were verified. Moreover, the results can help dispatch operators to locate the transmission lines with the highest congestion risk.