In a device for separating a synchronous signal in a video signal, a capacitor receives the video signal to obtain a coupling signal, a level determining circuit receives the coupling signal and compares a voltage level of the coupling signal with a number of reference voltages. The reference voltages define several reference voltage ranges, one of which is a predetermined reference voltage range. The level determining circuit outputs an adjusting signal according to a reference voltage range corresponding to a minimum voltage level of the coupling signal within a predetermined time period. A level adjusting circuit has several current sources for receiving the adjusting signal and thus controls the current sources to adjust a DC level of the coupling signal. A synchronous signal separating circuit separates the synchronous signal from the coupling signal when the minimum voltage level of the coupling signal is substantially within the predetermined reference voltage range.