Difference frequency laser used for the study of atmospheric molecule isotope abundance requires stable frequency and continuous idler radiation. A difference frequency generation (DFG) system is established based on a tunable continuous wave (CW) Tisapphire laser and a CW single-frequency NdYAG laser. In the DFG system, frequency stabilization method of iodine molecule Doppler-broadened absorption combined with digital proportion integration differentiation (PID) feedback control technology is adopted to stabilize the wavelength of infrared laser arised from the DFG system, while MgOPPLN is used as the frequency doubling crystal. The NdYAG laser frequency shift is stabilized within 1.2 MHz/h and the stability is 4.26×10-9. The experimental results show that the frequency shift within 1 h of NdYAG laser decreases rapidly with the increase of the voltage loaded on piezoelectric ceramic (PZT), but smaller shift is not achieved beyond 1 V; better stability is not achieved by changing the frequency of modulation voltage. NdYAG laser with stabilized frequency is used to generate difference frequency laser near 3.42 μm. By detecting low pressure absorption line of CH4, the 6.9 MHz line width of this difference frequency system is derived. The results provide not only an important reference for laser frequency stabilization by this technology, but also a stable difference frequency laser for trace gas detection.