In order to realize the inner focusing and meanwhile ensure the excellent image quality of the wide spectrum system, the position chromatic aberration and secondary spectrum in the wide spectrum optical system are corrected through the reasonable selection of materials, and a kind of design method of the wide spectrum optical system with inner focusing is proposed. The mathematical model of the achromatic with inner focusing is established, and the formulas that the system design needed are deduced. Combined with the proposed mathematical model and the derived formulas, a wide spectrum optical system with a focal length of 90 mm, the F number of 2.8 and the function of inner focusing is taken as an example. The result shows that in the range of 420~900 nm, the system can correct the chromatic aberration of the target at the distance of 0.2~200 km. The correctness of the design method and mathematical model of the inner focusing wide spectrum optical system is verified.