Many ways have been practiced in designing and fabricating micro spectrometer, each has its own characteristics distinguished by different spectral resolution and other performances. Micro spectrometers applied presently with high spectral resolution usually adopt optical systems with dispersive components being diffractive gratings. The development of micro spectrometer is reviewed briefly, and typical optical systems are analyzed and discussed, such as optical system based on Roland circle and that based on Czerny-Turner structure. The techniques and methods as well as the experiences of improving micro spectrometer's spectral resolution are summarized, especially the tradeoff between miniaturization and spectral resolution, and the balance between spectral range and spectral resolution are considered. This summary gives a outline of the optical systems' characteristic of dispersive micro-spectrometer, and it can be a reference for new micro spectrometer's designing.