To investigate the impact of pump structural parameters on performance and internal flow characteristics, experiments were conducted on hydrogen circulation pumps with different structural configurations. Additionally, computational fluid dynamics（CFD）simulations were performed to compare results when using air and hydrogen as working fluids, with an in-depth analysis of the internal flow field and temperature distribution. Results show that HP-2 can achieve the same pressure increase and flow rate as HP-1 can at lower rotational speeds but with higher power consumption. The simulation trends for both air and hydrogen were consistent, suggesting that air can be used as a substitute for experimental studies. HP-2, with a smaller impeller diameter-to-width ratio, exhibited a higher proportion of the flow channel occupied by the blade passages, which weakened momentum exchange between the impeller and the flow channel. This resulted in a lower proportion of medium-to-high velocity regions in HP-2 compared to HP-1. The increased axial clearance at the volute tongue region also had a more significant impact on the inlet flow. Despite similar temperature distribution patterns under different operating conditions, the temperature difference between the inlet and outlet of HP-2 was 29% higher than that of HP-1. Moreover, at the same flow rate, HP-1 was 23% more efficient than HP-2. Additionally, HP-2 exhibited more chaotic velocity vector distributions, leading to increased secondary flow and mechanical losses. The maximum angular deviation of the velocity vectors from the radial direction within the impeller passages of HP-2 was approximately 10°higher than that of HP-1, resulting in greater impact losses due to stronger impingement on the impeller suction surfaces and outlet duct walls, ultimately reducing overall efficiency.