Abstract: China LEAd-based Research Reactor (CLEAR-Ⅰ) is designed for the development requirements for accelerator driven sub-critical system and the fourth generation of lead cold fast reactor, which can run in critical and subcritical patterns, also, it has good neutronics and thermal-hydraulic properties and inherent safety feature. CLEAR-Ⅰ simulator is a full-physical process simulation system which is developed for CLEAR-Ⅰ. The simulator can be used for check of the design scheme, verification of the control program, simulation of the operation condition and so on. The thermal-hydraulic models running in critical state were developed by RELAP5-HD. Then models, including one dimensional models and three dimensional models in steady and transient states were performed. The simulation results are consistent with the data that the reactor designed, which validates the thermal-hydraulic models of CLEAR-Ⅰ based on RELAP5-HD developed in this paper. So the models can be used for researching on reactor control scheme further.