Spectrometer is a precision optical instrument，which can accurately measure the angle between two rays in the horizontal plane rather than in the vertical plane．In order to solve this problem，the spectrometer is added a sector gear and a solution platform to become a“polarimeter”and then to measure the optical rotation of solutions．By changing the concentration，length and temperature of the solution as well as the wavelength of incident light，the specific rotation is determined for the sucrose and fructose solution． It is found that experimental errors are less than 2．0%，indicating the high accuracy of the modified instrument． At the same time，compared with the traditional light of sodium lamp，the laser light is helpful for the more clearly experimental phenomena，easier operation，and therefore better teaching effects．