In this paper, we present a new kind of surface plasma resonance (SPR) sensor. It comprises three layers, an upper coupling prism, a middle metal layer, and a bottom semi-infinite two-level atom medium. Based on theoretical analysis on this SPR system, it is found that there are some new phenomena due to the coexistence of two resonant effects, i.e., the surface plasmon resonance and the resonant quantum transition between atomic energy levels. An interesting phenomenon is that the reflectivity of the incident light is very sensitive to the shift of atomic level induced by external field. This phenomenon is absent in ordinary SPR systems. Because the shift of atomic level is determined by external field, ultimately, the reflectivity of the incident light is sensitive to the external field. In this paper calculations are presented in the case where the external field is a magnetic field. It is found that the reflectivity of the incident light is very sensitive to the magnetic field. This property can be used for measuring a very weak magnetic field near surface of matter , thus it can be used for developing a new detection technology.