To suppress the optical interference fringes generated by dense-pattern optical multipass cells and enhance the detection accuracy and sensitivity of gas sensors, this work proposes a data averaging method within integer multiples (100) of the free spectral range to suppress laser optical interference fringes, in which the detection accuracy,sensitivity, and signal-to-noise ratio (SNR) for trace gas can be improved by investigating optimal average number of data points. Experimental results demonstrate that compared with the baseline standard deviations of 0.019 arb. units and 0.063 arb. units of the second harmonic signal obtained by the conventional interference fringe suppression method, those obtained by the porposed novel interference fringe suppression method 0.003 arb.units and 0.0066 arb.units respectively, increasing 6.33 times and 9.55 times correspondingly. The SNR of the novel method is increased by factors of 4.24 and 6.48 respectively. The detection sensitivity is 0.0008 ug/m^-3, increased by a factor of approximately 3.75. The detection accuracy is 0.49 ug/m^-3, increased by about 1.5 times. It is indicated that this proposed mthod can effectively mitigate the influence of optical interference fringes on measurement outcomes.