A phase-modulation surface plasmon resonance(SPR) sensor based on Mach-Zehnder configuration is presented and studied. Refractive index information of sample is extracted by phase substraction between the respective interference results of p-polarized and s-polarized light. Compared with the traditional phase-modulation SPR sensors, this design reduces the influence of environment noise and therefore greatly improves resolution. The system can distinguish 0.01° phase difference at least, which is equivalent to 10-5 RIU (refractive index unit). This system is employed to detect the reaction of biological molecules, and a resolution as high as 160 moleculars (IgG) per square microns is obtained. This study provides a certain reference to phase-modulation SPR sensors in application of bio-medical research.