Abstract: High temperature superconducting magnet system is typically composed of coils wound from composite superconducting tapes. In actual operation, the quench may occur in coils resulting from the local heat source, local degradation, overcurrent and other factors, and the whole structure may undergo elastoplastic deformation or even local damage under action of thermal stress and electromagnetic force. Due to the randomness and multi-factor triggering characteristics of local quench, the large aspect ratio for composite tapes with excessive compositions, and the electromagnetic-thermal-mechanical coupling and nonlinear characteristics, it is very difficult to accurately predict the electrothermal instability and mechanical behavior of composite superconducting coils. Most existing numerical studies reduce degrees of freedom by finely discretizing the local quench region and homogenizing the remaining region, which cannot handle random quench situations and provide mechanical information for each material layer in the quench propagation region. Based on the magnetic field-magnetic scalar potential equation, heat conduction equation, and elastoplastic mechanical equation, this paper develops an efficient and accurate analysis method for the electromagnetic-thermal-mechanical coupling behavior of superconducting magnets by combining the nonlinear power-law model and nonlinear stress-strain relationship. This method uses magnetic field equation to simulate the electromagnetic behavior of conductive regions such as superconducting layers, copper layers, silver layers, and Hastelloy layers, while magnetic scalar potential equation is used to simulate the electromagnetic behavior of non-conductive regions such as insulation layers and surrounding liquid nitrogen. By adopting a thin-cut method and introducing the discontinuity for magnetic scalar potential, the non-conductive region can be transformed from multiply connected to simply connected to obey Ampere’s law. Its calculation speed can be nearly 4 times faster than the current mainstream magnetic field method. In the multi-field coupling analysis of superconducting pancake coil, this method is used to discuss in detail the quench mechanism of the coil structure and the elastoplastic evolution of each material layer under multiple factors such as the local heat source, local degradation, and overcurrent process.