Abstract: A third harmonic conversion experiment are reportedusingtwo φ80 × 14mm KDP crystals, with the "polarization-mismatch scheme" in φ70mm output of Xingguang laser facility. When the input intensity to doubler is about 1GW/cm², the maximum external energy conversion efficiency we obtained for second harmonic conversionis 76.9%,for third harmonic conversion is 66.8%.The stability and reproducibility of the conversion are very good.Experiments also are performed for some main factorswhich affect third harmonic conversion efficiency.