The rubidium atomic magnetometer based on nonlinear magnetooptical rotation (NMOR) effect can obtain the Larmor frequency and external magnetic field value through the free relaxation sinusoidal oscillation signal with specific time length, thus, interceptlength of relaxation signal can affect the performance of the atomic magnetometer. In this paper, the influence of the interception time of the free relaxation signal on the sensitivity and the measured magnetic field was studied near the background magnetic field of 500 nT. The measurement results show that the sensitivity calibrated by the noise power spectrum decreases gradually and reaches a stable value of 023 pT/Hz1/2with the increase of the interception time, the measurement of the alternating magnetic field shows that the field values measured according to the free relaxation signals with long interceptionlength represent the average results. Therefore, the high sensitivity index of the NMOR atomic magnetometer in this paper is obtained to some extent through mathematical processing to average the fluctuation of the magnetic field value. This method can be used for reference to calibrate the sensitivity of other atomic magnetometers or experimental devices and analyze the measured magnetic field values.