A Collins formula method with a scaling factor between the target and source plane is proposed for laser propagation in optical system design, which can be used to evaluate laser optical system performance and tolerance analysis. The laser propagation in optical systems can be calculated by the Collins integral formula, and an angular spectrum method is derived by coordinate substitution. It is introduced a scaling factor m, making the choice of the observation plane more flexible and the calculation more accurate. A laser optical system is designed, and its tolerance analysis is conducted by the angular spectrum method. The evaluation criterion is the laser spot radius in the far-field, which is defined by 86.5% power in bucket. The radius of the laser spot in 90 m distance is from 0.8 to 1.4 mm by the tolerance analysis, which the ideal expectation is 0.92 mm and the experimental result is 1.01 mm. In the distance of 47 m, the radius is from 0.42 to 0.73 mm by the tolerance analysis, which the ideal expectation is 0.48 mm and the experimental result is 0.46 mm. The experimental results agree with the results of the tolerance analysis well. The focal shift for laser propagation in optical systems is validated. The experimental results confirm the calculation and they prove the use of the method in laser focus optical system design.