The accelerator-driven subcritical reactor system (ADS) is a kind of nuclear reactor which can burn minor actinide waste products produced from conventional reactors with inherent safety features. In this paper, the thermal-hydraulic model and a corresponding program for a 10 MW helium-cooled experimental ADS are presented. Through the analysis of the heat transfer mechanism in ADS, the thermal-hydraulic model of ADS was built, in which the solid domain is simulated with three-dimensional heat conduction model and the fluid domain is simulated with the one-dimensional quasi-static model. In order to analyze the transient characteristics of ADS with cooling system, a RELAP5–TRCAP coupling model for the cooling system was established, in which the decay heat of the target and core is considered. The results of steady condition and transients demonstrate the effectiveness of the transient model.