The invention discloses a method and a system for evaluating an attack hazard of a source network load system. This method analyzes the application scenarios, control functions and purposes of different control modes of source network load system, and forms different consequence evaluation indexes according to different control modes by using the influence parameters of power system security and stability evaluation indexes commonly used in power system and the specific purposes of each control mode. The method can depict the power loss caused by malicious attack on the source network load communication system with concrete numerical value, compare the functional difference between the normal operation scenario and malicious attack scenario of the source network load communication system,and timely maintain the attacked target, which lays a foundation for improving network security and making defense decision of the system.