Abstract: The vertical slot fishway is one of the main forms of fish migration assistance structure in the application of hydraulic engineering.Compared with the traditional fishway（eg.weir-flow fishway,Daniel fishway）,this kind of fishway is easier to create flow patterns that facilitate fish migration,and it’s suitable not only for small streams but also for large watersheds. Furthermore,it is eco-friendly to all kinds of fish species,including those with weak swimming ability or small size. As an important parameter in fishway design,slope gradient directly affects the hydraulic characteristics in the chamber of the fishway and is also related to the construction cost. In the present study,based on the RNG k-ε model derived from Navier-Stokes equations,the effect of the slope gradient of the vertical slot fishway on the velocity and total kinetic turbulence is studied numerically. Moreover,the variation of velocity and turbulent kinetic energy distribution with different slope gradients is systematically studied. The results indicate that,with the slope gradient of the fishway increases,the recirculation zone of hs is reduced by the extrusion range of the jet,and its value decreases from the initial 55.11% to 15.22%,leading to a significant reduction of the left return zone,which is very unfavorable for fish to rest during migration. The flow pattern in the hb plane,on the other hand,is less affected by the slope gradient and does not change much. The flow field is less affected in h_s and h_b planes when the slope changes from 1.5% to4.2%. However,when the slope gradient increases to 6.67%,the area of the low-speed recirculation zone in the hs plane decreases significantly compared with that in the h_b plane,due to the inflow of the high-speed mainstream. Especially for the left recirculation zone,the size decreases by nearly 20% while the slope gradient increases to 6.67%,and the simultaneous change of flow velocity and flow pattern reflect the three-dimensional characteristics that emerge under this kind of slope gradients. The maximum flow velocity at the vertical slot varies with the slope and exceeds 2 m/s with a slope gradient of 8.33%. Under this kind of slope gradients,the difference in flow velocity between the main flow area and the recirculation area is extremely increasing,and the maximum velocity in the main flow zone increases from 0.84 to1.78 m/s. However,the minimum flow velocity and the maximum flow velocity in the recirculation area are less affected by variation of the slope gradient. The magnitude and distribution of turbulent kinetic energy at 1.5% and 4.2% are more suitable for fish migration than the other two larger slopes. The results also indicate that the hs plane has fewer areas with high turbulent energy unsuitable for fish migration（＞0.05m2/s2）at larger slopes than hb due to the dispersion of the mainstream. Meanwhile,the average turbulent kinetic energy also increases with the slope gradient,and the corresponding maximum value reaches 0.035 1 m~2/s~2at the slope gradient of 8.33%.