Abstract: The distributed optic fiber temperature sensing system has excellent application prospect in dam leakage monitoring for its advantages such as low cost and quick distributed temperature measurement. However, the deficiency of theoretical and experimental studies limits the use of the system in qualitative analysis and leads to the lack of quantitative calculation. By using the temperature data detected by the system under non-heated and heated conditions respectively, a mathematical physical model is established to describe the relationship between temperature variation of ambient rock-soil and leakage velocity according to the theory of heat transfer balance. The determinate method for the leakage velocity is proposed in accordance with flow regime as turbulent and laminar flows. A case study is provided to prove the validity of the proposed model. The model provides a quantitative analysis approach for the wide use of the distributed optic fiber temperature system in dam leakage monitoring.