The invention provides an operation overvoltage calculation method based on the combination of electromechanical transient state and electromagnetic transient state, includes such steps of 1, calculating equivalent power source parameters considering actual power network operation state and mode; 2, establishing an overhead line model with distributed parameters; 3, establishing circuit breaker closing resistance and shunt reactor models; and 4, calculating 2% statistical operating overvoltage (i.e., greater than 98% value); After the above calculation steps, the value of line operating overvoltage considering the actual network operating state and mode can be obtained. In addition to the influence of line parameters, the value of line operating overvoltage calculated by the method also takes into account the effect of line shunt reactor, neutral point small reactance and circuit breaker closing resistance, and the influence of actual operation state and mode of power network.