Abstract: Aiming at solving the problems of low-precision and limited applications of the existing mechanical parameter identification methods for permanent magnet servo systems, a novel mechanical parameter identification method based on the orthogonality of trigonometric functions was proposed, which could identify moment of inertia, viscous friction coefficient and Coulomb friction coefficient. This method applied two sinusoidal speed commands of the same angular speed but different amplitudes. The sine and cosine functions multiplied with the electromagnetic torques were integrated separately. Based on the orthogonality of trigonometric functions, the expression of the moment of inertia and two linearly independent equations containing viscous friction coefficient and Coulomb friction coefficient were separated, which were all independent of a constant load torque. This method realized a decoupling between the moment of inertia and the friction coefficients under a constant load torque, and then improved the identification accuracy. The proposed method can be applied for applications, such as electric drives of cranes, hoists etc., carrying a constant load. Comprehensive simulation and experimental results verify the effectiveness of the proposed method. And the factors affecting the identification accuracy were analyzed.