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• A systematic thermal-hydraulic analysis code named TACS was developed. • The reliable calculation performance of TACS was verified and validated. • Both stead and transient thermal-hydraulic characteristics of HCOTSG were simulated. • The heat transfer process of HCOTSG was described from TACS simulation result. Due to the compact structure, the high heat transfer efficiency and the powerful ability to withstand the stress caused by thermal expansion, Helical Coil Once-through Steam Generators (HCOTSGs) have been widely used in recent years, especially as the important equipment of Small-medium Modular Reactors (SMRs). In this study, by adopting the finite difference method, the fixed boundary method, the staggered grid to discretize the control equations and the solution regions, and using the GEAR algorithm to carry out the numerical solution, a systematic Thermal-hydraulic Analysis code for HCOTSG called TACS was developed. Compared with the design values and the calculation values of the ONCESG code of the Marine Reactor X (MRX) steam generator in Japan and the System-integrated Modular Advanced ReacTor (SMART) steam generator in Korea, the correctness of the methods and the models used in the TACS was verified. Based on the TACS, the systematic thermal–hydraulic characteristics of the HCOTSG at full load were studied with a simulation object of the steam generator of the International Reactor Innovative and Secure (IRIS). The results showed that the length of each heat transfer zone is greatly different, so heat flux changes sharply along the tube length. Secondary thermal resistances determine the change of total thermal resistance, decrease at first and suddenly rise when dry-out happens. Frictional pressure drops are the main part of secondary pressure drops, accounting for 90% of the pressure drop in the whole flow process. Besides, the transient processes in HCOTSG with the step and linear disturbances of entrance parameters were simulated by the TACS, which conformed to the analysis of the thermal–hydraulic parameters. The final stable states of these processes are the same under two disturbance modes, but the changing processes caused by linear disturbances are gentler. [ABSTRACT FROM AUTHOR]