A mounting configuration is presented to solve the loss of the third harmonic generation (THG) efficiency due to the gravitational effects. The analysis result of finite element model shows that the loss of the THG efficiency is nearly absent due to the gravitational effects when the original surface profile of the KDP crystals and mounting configuration is an ideal plane. Meanwhile, numerical simulation calculations are performed to analyze the influence of fabrication errors on the THG efficiency. Furthermore, the machining requirements of the peripheral milling have been discussed. The loss of the THG efficiency is nearly same under the different departure angles. The mounting configuration with the random error is acceptable, but the convex error and concave error should be controlled below 5 μm to realize efficient third harmonic output.