The invention relates to a blowing type gas water heater constant temperature control method, which is characterized in that after the water heater is successfully ignited, a controller calculates the current thermal load Q according to the water flow V, water inflowing temperature T0, water outflowing temperature T1 set by a user, calculates and controls the air quantity S output by an air flower according to the thermal load Q, guarantees the flame not to be blown out, and causes the heater to work when the gas supply is low. The blowing type gas water heater constant temperature control system comprises a central controller, the air blower, a gas proportional valve, an igniter, a water flow sensor, a water inflowing temperature sensor and a water outflowing temperature sensor; after the igniter is successfully ignited, the controller calculates and controls the air quantity S output by the air blower according to the thermal load Q; when the real water outflowing temperature T11 is less than the set water outflowing temperature T1, the controller controls the gas proportional valve to increase the openness of the valve thereof and the air demand of the gas passage so that the real water outflowing temperature T11 reaches the set water outflowing temperature T1. The invention causes that the heater can work when the gas supply quantity is low, can rapidly recover the constant temperature state when the gas supply quantity recovers, thus being convenient for users to use.