The invention discloses a black level eliminating method, which comprises the steps of receiving a first level signal and a second level signal; starting a first time of counting when the first level signal and a ramp signal are received, and stopping the first time of counting when the amplitude of the ramp signal is equal to the amplitude of the first level signal so as to acquire a first count value; starting timing when the second level signal and the ramp signal are received, starting a second time of counting after delaying a predetermined clock period, and stopping the second time of counting when the amplitude of the ramp signal is equal to the amplitude of the second level signal so as to acquire a second count value, wherein the predetermined clock period is a black level value of a dark pixel unit; and acquiring a converted digital signal according to the first count value and the second count value. According to the method, black level is eliminated while conversion of the second level signal, and the time required by subtracting a black level value from an analysis result of an effective pixel unit in a digital circuit is saved.