It is efficient， low-cost and flexible to employ computer software for analyzing and verifying operation behaviors of protection devices under certain fault scenarios as well as designing new protection principles and tuning protection operation characteristics. With the solution of building protection modules inside ATP/EMTP software， a closed-loop software simulation platform is developed for studying protection principles and dynamic characteristics of computerized protection devices. It makes use of ATP/EMTP's capability of accurate electromagnetic simulation and is not subject to the limitation of the models coming with the ATP/EMTP software. The platform is easy to use and the simulation results are accurate and reliable. The platform is capable of generating intuitive information for protection characteristic quantities and operation logic in both text and graphic modes. As a functional extension， the platform can also take output data of ATP/EMTP as inputs to perform open-loop simulation， in compliance with actual protection device testing procedures.