Optimization of unit start-up sequence is studied considering line start-up time, switch line failure and unit start-up time. Firstly, line start-up time model is built based on line remote operation and probability of success operation, and line start-up time expectation is calculated as line weights. Secondly, unit start-up time, output characteristic and phase operation capability are studied to propose unit limit factor in the restoration. Then, Dijkstra algorithm is used to calculate the shortest path, unit comprehensive priority index is defined. Aiming at units rapidly start up, the units restoration optimization algorithm is put forward guiding by greedy thought to calculate the best restoration sequence, considering real-time line operation condition. At last, the proposed method is validated on the IEEE 39-bus power system. The achievements obtained in this paper have reference value and active meaning to the preplan formulation of black start and its online decision-making system development.