The invention discloses a steam supply system for a zero high-pressure heater in a coal-fired unit and an operating method. Saturated steam guided out of an outlet header of a startup separator passesthrough a regulation valve and a zero check valve to communicate with a heating steam inlet of the zero high-pressure heater. According to the steam supply system for the zero high-pressure heater inthe coal-fired unit and the operating method, the steam supply way of the zero high-pressure heater is optimized, the saturated steam guided out of the outlet header of the startup separator of a boiler replaces existing overheat steam guided out of a high-pressure cylinder to serve as heating steam of the heater, so that a heat-exchange end difference in the heater is sufficiently reduced, the heat exchange efficiency of the heater is effectively improved, and the operating economy of the whole unit is further improved. In addition, the heat-transfer end difference is reduced, so that the safety and reliability of the zero high-pressure heater are further improved. Meanwhile, the saturated steam is used as the heating steam, so that the heat stress of metal in the heater can be reduced,the operating safety and reliability of the zero high-pressure heater are sufficiently ensured, and the service life of the metal structure of the zero high-pressure heater is prolonged.