The primary task of making a black start plan is to select the to-be-restored unit.The main factors which have an impact on unit restoration are analyzed，especially the terminal voltage waveform distortion of black start unit caused by inrush current when no-load transformers are switched on.Based on the above analysis，the line weight，transformer weight and auxiliary motor weight are defined and the concept of success rate of unit restoration is proposed.The success rate of unit restoration and the capacity of units are considered to select the optimal to-be-restored unit during the black start stage.To reduce the computation burden，several units are selected as a candidate set，whose restoration paths are obtained by the algorithm of K-shortest paths.Simulation results of Shandong power grid show the effectiveness of the success rate of unit restoration and the rapidity of the proposed method.