The invention discloses an electromechanical transient method based relay protection fixed value online check method. The method comprises the following steps of (1) obtaining real-time measurement data of a power grid from an EMS, and establishing a power grid network algebraic model; (2) obtaining zero-sequence parameters and a protection fixed value of the power grid from a relay protection setting software database; (3) combining with a self electromechanical transient model parameter database, and solving a simultaneous algebraic equation and nonlinear equation through an electromechanical transient simulation method to obtain dynamic process data of each state variable of the system in a fault process and after the fault disappears respectively; and (4) performing check on the protection fixed value according to the dynamic process information of each state variable obtained in the step (3). According to the method, the accuracy of the short-circuit calculation result can be improved; online check and real-time early warning for the relay protection fixed value can be realized; and the automatic degree of the protection fixed value operation is improved.