The dynamic characteristics of active distribution networks (ADNs) need to be concerned if a large number of distributed generators are connected. A highly efficient and reliable simulation algorithm with good numerical stability is therefore essential for the stability analysis of ADNs. This paper proposes a novel dynamic simulation algorithm of ADNs based on the implicit projective method, which is a second order integration algorithm. The proposed method is an equivalent A-Stable method and the calculation efficiency is increased significantly compared with the traditional integration algorithms. It is especially suitable for the dynamic simulation and stability analysis of the ADNs with a large number of DGs. Case studies based on the IEEE 123-node test feeder show the feasibility and effectiveness of the proposed method, which is verified through the comparison with the commercial simulation tool DIgSI-LENT/PowerFactory and the traditional trapezoidal method.