Based on the teaching practice of semiconductor device physics，a simple law between the carrier

concentration and the Fermi level from the Boltzmann statistical law of non-degenerated semiconductor is summarized in this paper. When its carrier concentration is changed by one order of magnitude，its corresponding Fermi level should change by 60 meV. In order to understand this law deeply，this paper analyzes and calculates the important parameters，such as the quasi-Fermi level in non-equilibrium state，the work function of semiconductor，the threshold voltage of an MIS structure and the built-in potential of a PN junction. This facilitates students not only to deepen their perceptual knowledge in the process of mastering the physical knowledge of semiconductor devices，but also to develop and enhance the ability in analyzing and designing semiconductor devices.