Helical coil once-through steam generator (HOTSG) is widely used in the design of advanced small reactor because of its compact structure and high heat transfer efficiency. During the start-up process of HOTSG, there is a phase change process from single-phase water to superheated steam in the secondary working medium, and the thermal characteristics are complex, which may cause strong flow instability and wall temperature fluctuation, threatening the safe operation of the equipment. Based on the operation characteristics of HOTSG and reactor operation requirements, this paper puts forward a start-up strategy of HOTSG. By adjusting the change strategy of key operation parameters such as flow rate and pressure, the following advantages are achieved: shortening the start-up time and improving the economy of the plant; Realize calibration of reactor power in low-power and improve unit safety; Avoid strong flow instability and wall temperature fluctuation of HOTSG, and protect the stable operation of equipment. In order to validate the feasibility of this start-up strategy, the simulation model including HOTSG, start-up separator, condenser, feed water pump and other key equipment is established, and the dynamic simulation study of HOTSG start-up characteristics is carried out on this basis. The results show that the start-up strategy can make the HOTSG start-up successfully. By reasonably controlling the changes of flow rate, pressure and other parameters during the start-up process, the reactor power can be calibrated in low-power, and the unstable region of two-phase flow can be quickly passed through, which provides methodological guidance for the design and operation of HOTSG start-up process.