The practical demand for the use of Brillouin optical time-domain reflectors in engineering applications is constantly being raised: how to accurately determine the starting position of strain/temperature change, facilitate rapid repair, save manpower and material costs, especially in complex situations such as underground engineering and underwater engineering that cannot be directly observed, the precise determination of the starting position of frequency shift along the fiber optic line is particularly important. An improvement is made on the algorithm for obtaining spectrum in Brillouin optical time-domain reflectometer based on peak finding algorithm and short-term Fourier transform. A new quadratic peak finding algorithm is proposed, which calculates the starting and ending points of temperature/strain by judging the peak situation in the Brillouin gain spectrum. In the case of a pulse width of 100 ns and a monitoring distance of 1800 m, the positioning error of the starting and ending points of the 50 m temperature range is 0.4 m, achieving a spatial resolution of 1 m at a detection distance of 2000 m. This algorithm improves the accuracy of the Brillouin optical time-domain reflectometer in locating the temperature starting point and length without increasing the computational time and optoelectronic devices. Compared with the Brillouin optical time-domain reflectometer using peak finding algorithm, the Brillouin optical time-domain reflectometer using secondary peak finding algorithm has stronger practicality and higher cost-effectiveness.