A multi-objective prevention strategy for grid cascading failures based on the screening of vulnerable branches is proposed in this paper for the phenomenon of cascading failures triggered by line faults in power systems. Firstly,an improved line safety index and system safety index are proposed,and the Thiel entropy index is introduced to assess the vulnerability of each line in the power system to quantify the impact on grid operational safety after a line fault being withdrawn from the operation. Then,several branches that have a seriously negative impact on grid operational safety after an initial fault being withdrawn from the operation are screened according to the line vulnerability ranking to establish a set of expected incidents. Finally,a cascading failures prevention and control model for power systems is established which takes into account the system safety index and grid voltage stability,as well as the asility of coping with various initial fault scenarios. The model is solved by using the non-dominated sorting genetic algorithm-Ⅱ (NSGA-Ⅱ). The simulation analysis based on the IEEE 39-node system shows that the proposed prevention strategy leads to a safe and reasonable generator output solution,and it demonstrates the importance of line load factor differences in the identification of vulnerable lines.