A dc-dc converting circuit and a method for controlling the same are provided. The dc-dc converting circuit includes an output stage, a mode detection circuit, a PWM signal generating circuit and a ramp signal generating circuit. The output stage provides an output voltage. The mode detection circuit provides a mode detection signal. The PWM generating circuit provides a time signal to the output stage. When the dc-dc converting circuit enters a continuous conduction mode from a discontinuous conduction mode, the ramp signal generating circuit provides a second ramp signal to the PWM signal generating circuit in a preset time according to the mode detection signal. The ramp signal generating circuit provides a first ramp signal to the PWM signal generating circuit after the preset time. A slope of the second ramp signal is greater than a slope of the first ramp signal.