A waterjet propulsion device is a new type of propulsion system whose inlet passage affects the performance of the entire device. To study the influence of the inlet passage with an inclined pipe section (IPS) on the hydrodynamic characteristics of the device, a numerical method was used to determine the influences of IPS's length L and rotational speed n on hydraulic performance and thrust characteristics. Results show that the head H of the device increases significantly from 3.29 m to 12.64 m as rotational speed n increases (700 r/min < n < 1300 r/min). Moreover, efficiency η increases from 61.37 % to 64.75 % and then decreases to 60.99 %. L exerts minimal effect on η, which reaches its maximum value when n = 900 r/min and L = 1.12 D (D is the outlet diameter of the inlet passage). Device thrust F gradually increases with an increase in n, and the increase rate continues to rise, while L has minimal effect on F. The results provide a reference for optimizing the inlet passage of a waterjet propulsion device.