An angular-spectrum method with a scaling factor between the observation and the source plane was proposed for the laser propagation in an optical system to evaluate its performance and to analyze its tolerance.As the laser diffraction propagation in the optical system could be calculated by the Collins integral formula,the formula coordinate was transformed and an angular spectrum method was derived by coordinate substitution.Then,a scaling factor was introduced to make the choice of the observation-plane more flexibility and the calculation of the laser propagation more accurate.A laser optical system was designed,and its tolerance was analyzed by the angular-spectrum method.The evaluating criterion was the laser spot radius in the far field,which was defined by 86.5% Power in Bucket(PIB).Experiments show that when the radius of the laser spot is 0.8—1.4 mm in a distance of 90 m,the expected value calculated by the tolerance analysis is 0.92 mm and the measured result is 101 mm.Moreover,when that is 0.42—0.73 mm in the distance of 47 m,the expected value and the measured result are 0.48 mm and 0.46 mm,respectively.Both the experiment results match the data of the tolerance analysis well.The focal shift for laser propagation in the optical system was validated by the tolerance analysis.The theoretical analysis by the proposed method is verified experimentally,which confirms the feasibility proposed method in design of laser focus optical systems.