The invention discloses a quick constant-temperature control method for secondary water use of a gas water heater, which comprises the following steps: S1, in the case of secondary water use of a user, the interval time deltat between a current water use point and a last water use end point is calculated; S2, whether the interval time deltat is smaller than or equal to preset time ts or not is judged; S4, a fan is started, and an air valve is opened; S5, the last heating load is obtained, the ignition load is calculated through the last heating load, ignition is conducted through the ignitionload obtained through calculation, and the maintaining time is t1 after ignition succeeds; S6, switching to use the last heating load or the sum of the last heating load and the adjusting load is conducted for combustion for a period of time t2; S7, the heating load is adjusted through a PID algorithm; S8, whether a constant-temperature state is reached or not is judged continuously, if so, the current heating load is memorized and the step S9 is carried out, if not, the step S7 is returned; and S9, combustion is maintained until a user turns off water. By means of the technical scheme, rapidignition heating can be achieved, and the user bath experience is improved.