Abstract: The computational fluid dynamics (CFD) was used to simulate the labyrinth-seal structure of right angle trapezoid with different spacing in this paper. Then, the comparative research was conducted including the effects of spacing on sealing performance, flow field distribution, pressure field distribution and control of flow velocity, and the effect of eccentricity on sealing performance was analyzed at the optimal spacing. The results show that increasing the spacing of the labyrinth-seal structure can effect ively reduce the leakage flow, improve the sealing performance, achieve better control of flow velocity and can also significantly improve the flow performance near the wall surface and enhance the cooling of the outer wall surface of fuel assembly. As the spacing exceeds 3.02 mm, the improvement of sealing performance is very limited, indicating it is impossible to continue to improve the sealing performance of the labyrinth-seal structure by increasing the spacing when the spacing exceeds a certain limit. It is necessary to select the reasonable spacing in structural design and engineering application. The eccentricity leads to the imbalance of flow velocity and pressure distribution in the flow region, and as the eccentricity increases, the unevenness tends to expand, which not only adversely affects the sealing performance, but also exacerbates the vibration of the fuel assembly.