Abstract: With the rapid development of flexible AC/DC distribution systems, the large-scale access of nonlinear elements such as power electronic devices and constant power loads make the transient stability problem of the system increasingly serious. Firstly,a mathematical model of AC/DC distribution system with constant power loads based on AC/DC and DC/DC converters connected in parallel under double-loop control is established, and a nonlinear Taylor expansion of the system is carried out.Secondly, the original high-order system is decoupled into a series of low-order subsystems based on the nonlinear decoupling theory. The dominant decoupled subsystem is identified and analyzed in terms of transient stability by using the phase-plane method and the inverse trajectory method, and the validity of the theoretical analysis is verified by physical simulation. Then, the concept of the nonlinear participation factor is introduced based on the principle of nonlinear decoupling, and the participation factor of the state variables is calculated, so as to put forward the analytical method of the control parameters affecting the stability under the constant power load perturbation, and further give the feasible domain of the control parameters. Finally, an AC/DC distribution system with double-loop control under constant power load is constructed by using PLECS software, and simulation and experiment verify the effectiveness of the proposed method.