Tunable diode laser absorption spectroscopy technology is a developing technollgy on new optical gas sensor in recent years, currently there are three research methods: harmonic amplitude method, first to second harmonic signal amplitude ratio method and difference method. The harmonic peak to average power ratio (PAPR) is proposed on the base of comprehensive analyzing on the features and disadvantages of those three methods. The performance of optical methane gas sensor with new method is stable, and the detection results of the prototype are as follows: response time of sensor is 10 s, in the range of 0～5×10-3 the detection precision and sensitivity are 5% of full scale and 2×10-5, respectively.