Based on cascade energy utilization, a hightemperature gas-cooled reactor (HTGR) combined cycle offers an efficient energy conversion scheme, but its complex system configuration, strong parameter coupling, and dynamic response mechanisms require systematic analysis. In this paper, a dynamic model of an HTGR combined cycle system is developed in Modelica and validated by comparison with reference results. A negative reactivity disturbance of -300 pcm is then introduced to investigate the system transient response, with further comparison between constant-pressure and sliding-pressure operation. The results show that the steam generator heat-transfer rate decreases by 10.8% under sliding-pressure operation, less than the 15.1% decrease under constant-pressure operation, indicating better heat-transfer matching. However, steam turbine power decreases by 15.8%, compared with 3.8% under constant-pressure operation. Therefore, constantpressure operation is more suitable for maintaining main steam pressure and steam turbine power, whereas sliding-pressure operation is more appropriate for partial-load heat-transfer matching and load following.