Abstract: The solving for the initial structure of continuous zoom optical system by using PW method was introduced. The structure was obtained by distributing the power and length between them for each element on the base of the exchange principle of object and image. The PW values were changing for choosing best glass, controlling aberration,calculating the shape of glass. Finally an mid-wave infrared zoom system(50 mm~200 mm) was designed according to this method, the system had 5 lenses with 2 aspherics, satisfied 100% cold shield efficiency with simple structure.Moreover, the initial structure was optimized with Zemax.Result shows that the continuous optical system has high image quality and high energy transmission, which can meet the requirements of practical applications. In conclusion, the PW solution has practical meaning for the initial structure of zoom optical system design.