Asymmetric property of wedge lens in 3 optical path which is used as frequency separation, and focusing element is considered to be an unfavourable factor for target alignment in inertial confinement fusion (ICF). Furthermore, the thickness of wedge lens in 3 optical path will lead to laser induced damage inevitably. For the purpose of scheme improvement of final optical assembly, types I and II noncollinear sum frequency generation in KDP crystal at room temperature are discussed based on nonlinear coupled wave theory. As illustrated by simulated result, in addition to type II collinear SFG used in ICF recently, 351 nm (3) waves can be generated by type I or II noncollinear SFG process. This method can realize color separations of , 2, 3 in far field without asymmetric element such as wedge lens and posses adequate tolerance of matching angle corresponding to the high efficiency conversion. As calculated, for type I SFG, when the noncollinear angle is in the interval from 0 to 19.99, phase matching condition can be satisfied in KDP crystal. The noncritical phase matching angle 3 is 90 and the corresponding noncollinear angle is about 19.99. The tolerance of mismatching angle is about 20 mrad. For type II SFG, the noncollinear angle interval that can satisfy phase matching process is about 0-13.55. Like type I SFG, there is also an noncritical solution in type II process whose matching angle is about (3) = 86.53. Because of the smaller effective nonlinear coefficient in this case, high efficiency conversion needs about 5 cm thick SFG crystal under 1 GW/cm2. Correspondingly, tolerance of mismatching angle is about 20 mrad. Because of the harsh tolerance of noncollinear angle between and 2 and for the purpose of compactness of final optical assembly, another method of noncollinear SFG is proposed: a piece of silica wedge with 12 wedged angle is mounted behind the SHG crystal in order to produce a 3.5 mrad intersection angle between and 2, and after type II noncollinear SFG process, , 2, 3 will be frequency separated in far field automatically by using thin lens. The tolerance of incident angle corresponding to high efficient conversion is about 1.0 mrad. This scheme can improve the the final optical assembly used recently.