The invention discloses a fast auxiliary decision-making method for electric power system on-line isolated island grid connection, and belongs to the technical field of electric power system operation and control. A main isolated island is selected according to load capacity, grid connection channels of other isolated islands and the main isolated island is searched, the channels are sequenced according to the grid connection load capacity expectation value of all channels in estimated unit time, safety stability estimation and auxiliary decision-making calculation for eliminating the safety stability problem are carried out on the channels in batches according to an index, and if a feasible grid connection channel exists, the calculation of other channels is ended, and an estimation and decision-making result of the feasible channel is output. According to the situation that a power grid is split into multiple islands, the grid connection channels are searched on line and sequenced, all kinds of safety stability problems occurring in the process of grid connection of all channels are automatically checked in batches, control measures for solving the safety stability problems are worked out, an auxiliary decision is provided for a dispatch worker to fast and safely recover power grid paralleling operation, and the vacancy that no related auxiliary decision-making technology exists in an existing on-line defensive system is filled up.