A control method for an LLC resonant converter includes the following steps. Firstly, the LLC resonant converter is enabled. Then, the sonant converter is operated in a first modulation mode, so that the magnitude of the output voltage is greater than or equal to an intermediate voltage value. When the magnitude of the output current is greater than zero, a determining step is performed to determine whether the magnitude of the output voltage is greater than a reference voltage value. When the determining result is satisfied, the LLC resonant converter is operated in the first modulation mode. When the determining result is not satisfied, the LLC resonant converter is operated in a second modulation mode, so that the magnitude of the output voltage is lower than the intermediate voltage value.