Abstract: It is a common practice to employ a helical coil steam generator in several small modular reactor designs because of its enhanced heat transfer efficiency and advantages on compact structure. RELAP5 is one of the widely accepted thermal-hydraulic system analysis codes for reactors. However, the build-in empirical corrections are only suitable for straight tubes, but not for the primary side and secondary side of helical coil steam generators. In this study, the thermal-hydraulic models for helical tubes and tube bundles were selected and implemented in RELAP5 to develop a code module for helical coil once-through steam generator (HCOTSG). The validation for thermal-hydraulic models was carried out by experimental data and code-to-code comparison. Based on the validated RELAP5-HCOTSG, the integral benchmark simulation against the steam generator of the international reactor innovative and secure (IRIS) was performed. Compared with the original RELAP5 calculation, the thermal-hydraulic parameters obtained by RELAP5-HCOTSG are in good agreement with the design values, which confirms the applicability of the developed code in this study for thermal-hydraulic analysis of HCOTSG.