The invention relates to a method and a system for screening an N-2 fault. An active power limit value for each branch in the power grid, active power variations before and after breaking of each remaining branch for a target branch and an active power value before breaking of each remaining branch for the target branch are acquired firstly; according to the above three values, an overload contribution rate for the target branch by breaking of each remaining branch is acquired respectively; the remaining branches are grouped according to the overload contribution rates, two branches whose overload contribution rate sum is no smaller than an overload contribution rate standard value are screened according to the grouped condition, and an N-2 fault is obtained. By adopting the scheme of the invention, each possible combination of two branches does not need to be screened, grouped screening is carried out according to the grouped condition, the huge amount of computation which is difficult to calculate caused by combination explosion can be avoided, and the computation amount for power grid risk analysis is greatly reduced.