Constraint condition and crystal temperature are two mainly factors that affect the performance of potassium dihydrogen phosphate (KDP) crystals. Under constraint conditions, the original phase matching condition is destroyed and third harmonic conversion efficiency is decreased as a result of thermal stress and thermal deformation caused by temperature variation. An analysis method of crystals thermal sensitivity is established to find the relationship between the temperature and phase matching angle. The finite analysis is used to obtain the distributions of thermal stress and thermal deformation, and then thermal-optic effect, stress-optic effect and thermal deformation are considered to calculate the change law of phase matching angle. The validity and feasibility of method are verified by using crystal constraint condition of SG-III prototype laser facility. The case indicates that the efficiency calculation results of method are better conforming to the actual change law of phase matching angle.