The invention discloses an implicit-projection-based method for simulating the stability of an active power distribution system. According to an active power distribution system stability simulation model with the rigidity characteristics, based on system simulation parameters and load flow calculation results, the implicit-projection-based method includes the steps that firstly, a plurality of steps of small-step-length integral calculation are carried out through an internal integrator, wherein the step length is h, and any explicit integral algorithm with the accuracy higher than the second order can be adopted; secondly, a step of large-step-length integral calculation is carried out with the step length Mh through an external integrator based on an implicit prediction-correction method according to the calculation result of the internal integrator. By means of the implicit-projection-based method, simulating calculation of system faults can be achieved, the implicit-projection-based method is a second-order accuracy algorithm and has the good numerical stability, a numerical stability domain of the implicit-projection-based method is hardly changed along with changes of the multiple of the step length of the external integrator, the performance of the algorithm is better than that of the explicit projection integral algorithm and that of a traditional implicit trapezoidal method, the implicit-projection-based method is suitable for rapidly achieving the aim of simulating the stability of the active power distribution system with the multi-time scale characteristics, and a foundation is laid for efficiently and reliably developing an active power distribution system simulation program.