3~5um mid-infrared laser displaying good atmospheric transmitting properties is mostly used as laser source, in many applications such as remote sensing for air pollution determining. And it is usually obtainable in Second harmonic generation (SHG) in CO2 laser. In the course of investigation, a homemade nonlinear optical crystal AgGaSe2, of the size 7×8×12mm3, was used for SHG in tunable TEA CO2 laser with different wavelengths. And, 12 coherent laser sources in 3~5μm from CO2 spectrum region 10.6μm~9.2μm were obtained. In our results, phase-matching angles aqre in good accordance with the crystal's cutting. Theory calculating on conversion was made as well, to compare with experimental data which present changes of energy conversion under pump's rising. And in the interest of enhancing energy conversion by means of peaking pulse through cutting tail, a plasma shutter, argon charged in body under normal air pressure, was effectively arranged in experiment. As a result, about 50nS width peak pulse was generated, and up to 10% energy conversion (2mJ, 5.3μm) was achieved against 1% without shutter, both pumping in 10P(20) line.