Brillouin optical time domain reflectometer (BOTDR) is a distributed optical fiber sensor with broad application prospects. For a given input light wavelength, the Brillouin frequency shift of spontaneous Brillouin scattering light has a linear relationship with both temperature and strain. The distributed temperature or strain can be obtained by measuring the Brillouin frequency shift (BFS) along the fiber. In a BOTDR, Brillouin power spectrum scanning is a commonly used means to obtain the BFS, and there has been two types: the optical frequency-difference scanning method and the electrical frequency scanning method. A wavelength scanning BOTDR is proposed based on the wavelength dependence of the BFS. With a tunable laser as the light source, the Brillouin power spectrum is obtained by scanning the input light wavelength. This method combines the advantages of the optical frequency-difference scanning method and the electrical frequency scanning method. The feasibility of this proposed method is demonstrated experimentally. By use of this method, a temperature accuracy of 2.2 ℃ along a 23.4 km sensing fiber with a 5 m spatial resolution is achieved.