In order to quickly construct the real-time simulation model of the distribution network and reduce the human error, it is necessary to study the method of converting the CIM-based distribution network, which comes from DMS, into RTDS simulation model. After discussing the difference and relationship between CIM and RTDS model, a number of key technologies of automatic conversion from the CIM-based distribution network to RTDS simulation model are proposed. In the process of topology transformation, a method of generating coordinate position based on spanning tree is proposed. On this basis, a method based on minimum independent closed loop is proposed for closed loop and concentrated topology circuit. And two evaluated indicators, i.e. the connection branch ratio and dispersion, are used to determine the circuit topology that applies to the latter method. Finally, a software about corresponding model conversion is developed. And a real distribution network is converted to RTDS simulation system by the software, which verifies the effectiveness of the software. This work is supported by National Key Research and Development Program of China (No. 2016YFB0901300) and Technology Project of State Grid Zhejiang Electric Power Company (No. 5211DS15002R).