The theory of tandem SHG and the requirement on the relative orientations of two KDP crystals are discussed in detail. Using two 1.4-cm-thick type-II KDP tandem crystals, an overall energy efficiency higher than 60% was obtained in our experiment with the incident intensity ranging from 0.33GW/cm2 to 0.67GW/cm2. The laser beam had a diameter of 42mm. The experimetal results were in agreement with theoretical calculations. The measured performance of the tandem doubler (L1 = L2 = 1.4cm) was compared with those of simple crystal doublers with thicknesses of 3 cm and 1.4 cm respectively. The tandem doubler was found to have a combined merit of both the thick crystal doubler and the thin crystal doubler.