In photoacoustic spectrum measurement, optical chopper is commonly used to modulate frequency of light source output signal, however, the use of optical chopper inevitably leads to the increase of noise and cost of the system. On the basis of the tunable characteristics of distributed feedback laser, a dual wavelength modulation metod of the laser is proposed. Using photoacoustic spectroscopy experimental platform with optical chopper modulation laser system, we carry out the detection sensitivity experiment of methane. The results show that the detection sensitivity of optical chopper to methane is 52.3×10-6, and that of the dual wavelength modulation laser system to methane is 40.2×10-6. The dual wavelength modulation method avoids the use of optical chopper, decreases the system noise, and improves the system sensitivity.