The invention discloses an automatic control method for the heating rate of saturated vapor in a start-up heating stage of a thermal power unit. The method comprises the steps that a coal mill and a coal feeder are started, the material level of the coal mill is established, the minimum capacity and blast capacity of the coal mill are set, and coal powder is conveyed into a boiler for combustion; combustion lasts for a preset time; whether completion conditions of the start-up heating stage of the to-be-controlled thermal power unit are met or not is judged, and if not, the saturated vapor heating rate change situation is judged; if change is too slow, the capacity and the blast capacity of the coal mill are increased within a first designated time period according to a first designated capacity and blast capacity change rate; and if change is too fast, the capacity and the blast capacity of the coal mill are decreased within a third designated time period according to a second designated capacity and blast capacity change rate. According to the automatic control method for the heating rate of the saturated vapor in the start-up heating stage of the thermal power unit, the heating rate of the saturated vapor in the start-up heating stage of the thermal power unit can be automatically controlled and maintained at a high value not exceeding an upper limit value, the labor intensity of operators is relieved, and the safety and economical efficiency of the start-up heating stage of the thermal power unit are improved.