The helical-coiled once-through steam generator (H-OTSG) is a key equipment in small modular reactor (SMRs) to be used in heat exchanging between primary side and secondary side. In this study, a 1D Code HTAC (H-OTSG Thermo-hydraulics Analysis Code) is developed to simulate the heat transfer characteristic of H-OTSG and used to prepare the experimental study in future. The H-OTSG is composed by 74 helical tubes which is divided into 10 layers according to the coil diameters. Firstly, a 1D code of H-OTSG is developed with the tube-side thermal-hydraulic model proposed by the present authors and the shell-side model using the thermal-hydraulic model of straight-tube bundle in cross flow. Secondly, 10 inclined straight pipes with different coil curvature ratio are stablished in the 1D code to respectively simulate the helical tube bundle of 10 layers and the shell-side of H-OTSG is model with a vertical annulus. Furthermore, the sensitivity study on the node number is conducted and 99 nodes for each pipe and annulus are fine enough. Thirdly, the simulated result shows the 1D code can used to predict the steady-state heat transfer rate of the H-OTSG under various thermal power. Finally, the sensitivity analysis of the system parameters on the heat transfer rate of H-OTSG is conducted, and the result shows the decrease of tube-side pressure and the increase of shell-side inlet temperature can rise the average HTC of H-OTSG.