Usinga 3-cm-thick type II KDP crystal at input intensity of 0.27GW/cm2, an overall doubling energy conversion efficiency up to 61.5% has been achieved in our experiments. The 1.06μm laser beam has an aperture of 42mm and a divergence of ～0.5m rad. The temporal distribution of the intensity is nearly Gaussian. Experimental results are in agreement with theoretical calculation.