The invention relates to a method for selecting a reduced-order variable of a power electronic interface grid-connected system model reduced order, which can improve the adverse effect that the current application does not consider the reduced-order processing of different state variables aiming at different model reduced order errors. The method comprises the following steps of: 1, establishing an electromagnetic transient model of a power electronic interface grid-connected system; 2, solving a characteristic value of the system at a steady-state operation point, and introducing a characteristic value sensitivity method to analyze the correlation between the characteristic value and each state variable of the system; 3, extracting singular perturbation parameters corresponding to each state variable of the system, and establishing a singular perturbation model of the system; and 4, according to a given model order reduction error range, applying different dominant characteristic value selection modes to obtain the dominant influence state variable of the dominant characteristic value, and the remaining state variables are subjected to order reduction processing. According to the method, different state variables can be selected for order reduction processing according to the requirement of the model order reduction error, and the practicability and the effectiveness of the order reduction variable selection method are fully considered.