Abstract: In order to improve heat transfer efficiency of the traditional helical coil, the turbulent flow and heat transfer characteristics of a helical coil under uniform heating of water flowing are experimentally and numerically investigated. The main study focus is on the characteristics of local secondary flows in the axial cross-section perpendicular to the coil and their heat transfer effects between the working fluid water and the heated coil wall. It is found that the absolute value of the Nusselt number of the simulation results match well with the experimental results although local Nusselt number oscillation of the coil is discovered only in the experiments rather than in the simulation results. The reason may be that, though the secondary flows indeed exist and evolve irregularly along the coil axis, their effects on local heat transfer are almost negligible since the secondary flow velocity is relatively too small compared with the main flow velocity. To further confirm the characteristics of the secondary flows in the coil, a 360°external reverse loop is added to each 180°coil turning. It is found that the additional reverse loops increase the velocity of the secondary flows reflowing into the main coil and increase the number of the secondary flow vortices in a cross-sectional area.