The theory of photoacoustic spectrum is presented briefly in this paper. The optical fiber phase sensor is used to replaces traditional microphone, and the relationship between light phase and photoacoustic signal is discussed. Multiple-reflection system is used to enhance absorption effectively. The dye laser is used as light source to measure SO2 gas concentration. It shows that the sensitivity of 1.2×10-10 can be obtained. The combination of spectrum technology and optical fiber technology provides higher sensitivity, stronger ability to resist electromagnetic interference, avoiding fire and explosion. The gas probe has small volume, relatively fast response speed. The signal processing circuit has high ability to suppress noise interference. The experiments show that the sensor and its system match the requests of sensitivity, accuracy, and response time in detecting the gas concentration.