The waterjet propulsion is widely applied in the marine vessels over 30 knots, and the intake duct is considered as an essential component that strongly relates to the propulsion performance. This paper sheds light on the flow features inside an intake duct under mooring conditions by using the particle image velocimetry (PIV) technique with three-dimensional (3D) numerical simulations. The hydraulic loss gradually increase as the flow-rate increases. According to analyses via the Bernoulli equation, the hydraulic loss is composed of the frictional head loss ( h f ~ V 1.75 ) and the local head loss ( h j ~ V 2.0 ). A recirculation region is observed near the duct lower wall with a high-velocity flow near the upper wall, and subsequently a shear flow presents in the horizontal straight pipe with an obvious velocity gradient. Three-dimensional simulations demonstrate that the vortex pair is very strong in the recirculation region and then it gradually decreases as the fluid flows downstream. With the flow-rate increasing, the non-uniformity at the duct outlet firstly increases to a peak and then slightly decreases, while the perpendicularity at the duct outlet dramatically decreases to a minimum and then increases. This work not only reveals some physics of the waterjet propulsion under mooring conditions, but also promotes its efficient operation. Graphic abstract The hydraulic loss gradually increase as the flow-rate increases. The hydraulic loss is composed of the frictional head loss ( h f ~ V 1.75 ) and the local head loss ( h j ~ V 2.0 ). A recirculation region is observed near the duct lower wall with a high-velocity flow near the upper wall, and a shear flow presents in the horizontal straight pipe with an obvious velocity gradient. Numerical simulations demonstrate that the vortex pair is very strong in the recirculation region and then it gradually decreases as the fluid flows downstream.