In this paper, the second harmonic generation (SHG) of the TEA CO2 laser in an AgGaSe2 nonlinear crystal is realized. How energy of TEA CO2 laser, repetition frequency of pulse and focal position affect SHG is studied emphatically. The tuning curve upon incident angle is obtained and the maximal second harmonic energy 1.32 mJ is measured with the energy conversion efficiency 2.4% for total and 4% for main peak. Besides, the influence of pulse repetition rate (<10 Hz) on SHG conversion efficiency is analyzed. From the experimental results, it is found that the main factors restraining conversion efficiency are the crystal′s optical quality, the low power intensity tail of lasing pulse and the instability of pulse radiation. Specially, the thermal lens effect is intensified due to the long tail attached to pulse, which leads to decreasing of conversion efficiency if pulse repetition rate is enhanced.