Semiconductors play a important role in the contemporary age． In college physics experiment teaching，it is of great significance for students to deeply understand the band gap properties and measurement methods of semiconductors． The ultraviolet-visible diffuse reflectance spectroscopy method is an easy way to determine the band gap and energy band structure of semiconductors． It uses ultraviolet-visible diffuse reflectance spectroscopy and the Tauc Plot method to determine the band gap of the Ｒuddlesden-Popper semiconductor perovskite Srn+1TinO3n+1( n = 1，2，3，∞ ) ，and discusses the application of the Tauc Plot formula in transmission and reflection conditions．