In order to study the sensor of atomic interference magnetometer based on coherent population trapping principle and analyze its control variables, a mathematical model was proposed to formulate the relationship between system input variables and output signals. A mathematical model of the key variables of frequency discrimination signal contrast was also established and signal contrast experiments were performed to observe the atomic gas chamber temperature and laser input current, which are the key factors in this system. Experimental results showed that the average contrast of frequency discrimination signal was about 3%, which was in agreement with the order of magnitude of the model. The influence of the atomic gas chamber temperature and the laser current on the signal contrast was also in accordance with the results of the model. Moreover, the experiments also determined the range of dynamic operation points for an optimal signal contrast.