A quasi-integrated numerical model for the vertical slot fishway was established to investigate the allowable maximum water depth difference between the fishway inlet and outlet, particularly when the fishway is subject to insufficient inlet water depth. The mathematical model was used to numerically simulate the flow velocity, flow pattern and water surface line along the fishway. The results suggest that when the water depth at the inlet is insufficient, the water depth in the upstream section is consistent with that of the outlet, and the flow velocity is stable. The water depth in the middle section begins to drop slowly, and the flow velocity gradually increases. The water depth in the downstream section decreases sharply, and the flow velocity increases rapidly. There is a drop at the inlet and the maximum flow velocity is observed. It indicates that the outlet depth has a linear relationship with the allowable maximum water depth difference between inlet and outlet, i.e., the greater the outlet depth, the greater the allowable maximum water depth difference between inlet and outlet. The results also demonstrate that there is a linear relationship between the fishway slope and the allowable maximum water depth difference between inlet and outlet, which is that a steeper slope corresponds with a smaller the allowable maximum water depth difference between inlet and outlet.