The methods to produce mid-infrared lasers and the features of different mid-infrared lasers are introduced. Combined with the advantages of the second harmonic generation in CO2 laser, the performance of frequency doubling crystals in the CO2 laser are mainly discussed. The performances of the most widely used birefringent phase matching (BPM) and quasi phase matching (QPM) frequency doubling crystals are compared and analyzed. The problems and probable techniques for the development of second harmonic generation in CO2 laser are previewed.