A soft-start circuit and a method thereof are described. The circuit includes an amplifier and a voltage ramp generator. The amplifier has a first input end, a second input end, an output end, and a power source control end. The first input end is coupled to a reference voltage. The second input end is coupled to a feedback voltage. The output end outputs an output voltage, and the feedback voltage corresponds to the output voltage. The voltage ramp generator is coupled to the power source control end, and generates a ramp-up voltage. When the ramp-up voltage is lower than a threshold value, the output voltage rises with the ramp-up voltage. When the ramp-up voltage is not lower than the threshold voltage, the output voltage remains at a stable value. A surge current occurring during smooth soft-start or even in operation is thus prevented.