A high-performance quantitative detection distributed optical fiber sensing technology based on Rayleigh scattering is proposed and studied. Based on the phase-sensitive optical time-domain reflectometer (Φ -OTDR), a direct detection coherent optical time-domain reflectometer (COTDR) sensing system is set up, by combining the narrow-linewidth laser with microwave electro-optical modulation to obtain optical frequency-shift precisely. The sensing quantity (temperature or strain) can be obtained from the change of the Rayleigh scattering light interference pattern generated by the frequency shift of the light source. The sensing principle of the direct-detecting COTDR system is studied, with simulating and analyzing the COTDR sensing process and the correlation characteristics of the scattering signals. On the basis of theoretical analysis, the direct- detecting COTDR experimental system is built, and 25 km distributed optical fiber temperature sensing experiment is realized. The experiment results show that the COTDR experimental system can measure the temperature change of 0.1 ℃ at the end of the 25 km sensing fiber.