With a brief discussion of the physical mechanism of spectroscopic derivative, the successful applications are given in optical absorption and reflectivity study of GaxIn1-xAs/InP and GaxIn1-xP/Al GaInP multiple quantum wells . The effects of different orders of the derivative are compared with each other , and the difference is pointed out between this method and the curve-fitting me thod as well as the double modulation method. A conclusion is reached that the m ethod is practical and reliable in the optical study of wide band-gap quantum we lls.