A novel time-difference fluxgate sensor with weak sinusoidal and Gauss noise excitation is presented in this paper. Expression of the transition rate is derived according to the bistable peculiarity and the Fokker-Planck equation of the soft magnetic material. Relationships among transition rate, external magnetic field, amplitude of the excitation field, and noise intensity are discussed through numerical simulation. By converting the periodic transition rate signal to the square signal, the time difference between the high level and the low level of the square signal can be related with the external magnetic field. And the expression of the sensor's sensitivity is derived. Simulation results show that the sensitivity is inversely proportional to the amplitude and frequency of the excitation magnetic field, and the measuring range is proportional to the amplitude of the excitation magnetic field. Experiments have been carried out on a fluxgate prototype with a measuring range of ±10.7 A/m. The minimum sensitivity could achieve 9.8696 ms/(A/m), and the novel time-difference fluxgate can be of great interest in the detection of quasi-static extremely weak magnetic field.