In order to study the long-term frequency and voltage dynamics of interconnected power systems under cascading events， this paper suggests an appropriate mathematical model and its simulation algorithm with the network structure preserved. Similar to traditional dynamic power flow， the proposed model assumes that the system has survived the transient stability and has sufficient damping torque. Therefore， the relative swings among generator rotors in one area will be neglected in long-term dynamics; and each control area is recommended to have a uniform frequency， represented by its area center of inertia（COI）， with a simulation of the long-term frequency and voltage dynamics and the investigation on its effective control strategies. Computer test results of the IEEE 3-area 30-bus system show the validity and effectiveness of the proposed model and algorithm. The proposed simulation tool can be used to study long-term frequency and voltage stability control of interconnected power systems.