The invention discloses a short circuit protecting device adapted to a DC-DC converter with a soft-start function, and a voltage output terminal of the DC-DC converter increases an output current in segments in multiple time periods after the DC-DC converter is started. The short circuit protecting device including a voltage sensor connected to the voltage output terminal to sense the output voltage; a start timer generating a enable signal in a predetermined period after the DC-DC converter is started; a comparison circuit connected to the voltage sensor and the start timer and comparing the output voltage with a predetermined voltage according to the enable signal. The sum of the time period is a startup time of the DC-DC converter, and the startup time is longer than the predetermined period. When the output voltage is smaller than the predetermined voltage, a comparison circuit outputs a control signal to stop the DC-DC converter.