To improve the double-pipe heat exchanger with helical fins on outer wall of inner tube，the flow characteristic of helical duct with rectangular cross section was investigated numerically and experimentally.The computed velocity profiles with aspect ratio（the ratio of height to width of the cross section）of 3.5 were in good agreement with the measured profiles.The axial velocity，secondary flow，stream function and vorticity of the flow field were analyzed at different aspect ratio based on the orthogonal helical coordinate system.The maximum of the axial flow shifts toward the outer wall.The secondary flow is in two-vortex structure，in which the direction of the secondary flow in the upper section is clockwise while that is adverse in the lower section.There are two vorticity in opposite direction in the center besides the stronger vorticity at the boundary.As the aspect ratio increases，the maximum of the axial flow shifts toward the upper section，the two vortices of the secondary flow shift toward upper and lower boundaries respectively，and the vorticity in the center decreases.Therefore，in order to improve heat transfer further，for the double-pipe heat exchanger with helical fins on outer wall of inner tube，in which the heat transfer is mainly enhanced by the secondary flow，the secondary flow in the center of the section should be improved when the aspect ratio is large.