According to the design requirements of a submarine spherical robot, the viscous flow in curved rectangular ducts with different section dimensions and curvature radii is simulated numerically with CFD (computational fluid dynamics) under different flow patterns. Taking the centrifugal force effect into consideration, the variation of the secondary flow intensity and viscous resistance along with the changing parameters of curved ducts is studied intensively, drawing the conclusion that the hydraulic loss of a curved duct, whose aspect ratio is 0.75, is the lowest. Then, the flow structure of the curved duct is analyzed. Finally, the vortex intensity of outlet section is measured on a standard testing platform with the help of a kind of vorticity probe, verifying the correctness of numerical simulation. Thereby, the design and optimization of the pipes in the submarine spherical robot are completed by comparative analysis.