The invention discloses a method for identifying and optimizing a fragile line of an electric-gas coupling system. The method includes inputting original data, establishing an electric-gas coupling system optimization model, and processing to obtain initial operation state of coupling system; turning off the output power of each gas source of the natural gas network, calculating the gas source vulnerability, and carrying out weight processing to obtain a line coupling vulnerability factor; extracting a topological structure of the electric network and processing to obtain a line topological vulnerability factor; respectively disconnecting each power grid line to obtain a power flow distribution entropy so as to obtain a line operation vulnerability factor; comprehensively obtaining the comprehensive vulnerability of the lines, sorting according to the descending order, and the lines larger than a set threshold value are the fragile lines; and after the fragile line is identified, the transmission capacity of the fragile lines is improved through the thickened wire. According to invention, fragile line in electricity-gas coupling system can be rapidly identified, monitoring and protection of fragile line are enhanced, and probability of large-area power failure of power system can be effectively prevented.