Losses of light reflection of cesium lithium borate (CLBO) crystal with four frequency muliplication at different cutting modes is researched. Optimal Bruset cutting method of antirefelction implemented by laser multiplication frequency crystal is given. For nonlinear crystal, antireflection effect can not be got by the method which is suitable to laser crystal. The reason is that new frequency is not produced when light pass through the laser crystal and ploariztion states of the light are not changed. But new frequency is produced when light pass through nonlinear crystal. The vibration states of new frequency are always different from those of incident light. It is determined by matching conditions of phase and polariztion together. So the method of two pass light surface cutting in parallel at Bruset angle is not suitable to the crystal any more. By analysis, if the crystal is cut by two pass light surface at Bruset angle perpendicular each other, UV light antireflection effect can be got.