In order to improve the resolution of phase modulated surface plasmon resonance (SPR) measurement technology. The signal processing technology is optimized. In the differential phase SPR biosensor based on Mach-Zehnder configuration, the SPR phase can be extracted by comparing the phase difference between the interference signals of p- polarization and the reference signals of s- polarization. The 2- path interference optical signal is recevied by computer. According to the mathematical theory and signal processing knowledge, select and processing the interference optical signals. The phase difference of 2-path signal is analyzed at the same time. In the signal processing technology, the phase difference of ups and downs is 0.006°, and the sensitivity limit is 4.7×10-7 RIU.