Disclosed is an optical time domain reflectometer simultaneously sensing temperature and stress. The system is based on parallel detection of Rayleigh and Brillouin scattered light and includes devices such as a multi-wavelength laser source, a light-pulse modulator, a balance detector, a microwave amplifier, a high-speed data acquisition card, a coupler and a circulator and the like. In the optical time domain reflectometer, the frequency interval of wavelengths of the multi-wavelength laser source is arranged to be in a range of 9-12 GHz, which is equivalent to a frequency shift quantity of Brillouin scattered light in a fiber. A heterodyne coherent detection method is used to carry out parallel detection on Rayleigh and Brillouin scattered spectra and temperature and stress information is demodulated through a Landau-Placzek ratio (LPR) and Brillouin frequency shift distribution; and at the same time, coherent Rayleigh noises are reduced and superposition of the scattered spectra improves the signal-to-noise ratio of the scattered light. The optical time domain reflectometer is capable of realizing simultaneous temperature and stress sensing of a distributed fiber sensing system, improving the signal-to-noise ratio of the scattered light and improving the sensing precision and distance.