The optical system is the core part of a spectrometer, and it determines the basic properties and the volume of the entire instrument. Advantages and disadvantages of several optical systems are analyzed, and the asymmetric Czerny-Turner system is adopted. The resulting optical path optimized with Zemax has an overall resolution of 1.8 nm, and a measurement range of 200~900 nm. Relative error of wavelength is controlled to be below 0.05 nm, with cubic spline interporlation coefficients obtained from Matlab and HG- 1 mercury- argon used as the calibration light source. Analyses on data for comparision with the USB4000 fiber spectrometer show that both of the design and implementation are feasible.