The purpose of this work is to explore fluid turbulent flow and heat transfer characteristics in the single rectangular helical duct with different aspect ratios of cross section γ. Furthermore, better heat transfer enhancement effect of the jet on the rectangular helical duct can be obtained. Computational fluid dynamic software is employed to research the turbulent flow fields, secondary flow fields and heat transfer characteristics both in the single rectangular helical duct and the helical duct mounted with a jet pipe. The jet pipe is mounted on the outer wall of the helical duct with a certain angle. Four aspect ratios of the cross section γ as 0.625, 1.1, 1.6 and 2.5 respectively are selected for the rectangular helical duct. The results show that with the same cross-section area and flow rate, a four-vortex structure of the secondary flow can be found only in the helical duct at γ≥1.6 under high Reynolds number. Secondary flow pattern under other conditions is a two-vortex structure in the studied range. For a single helical channel, the larger the γ value is, the smaller the flow resistance is and the worse the heat transfer performance is. Heat transfer capacities of the four walls on the rectangular cross section are all improved after the jet is added to the helical duct. The larger the γ value is, the more significant heat transfer enhancement effect of jet can be obtained. The average value of the local Nusselt number on the heated wall (Nulocal)m can be up to 2.51 times than that of a single helical duct when γ=2.5 and the ratio of jet velocity to the mainstream velocity εj=3.55. The comprehensive enhanced heat transfer factor PEC2 of the helical duct area under the influence of the jet is between 1.05~1.21 when the influence of the increase of jet flow rate is considered.