The operation state of the water conveyance tunnel is closely related to the water condition, so determining the height of the liquid-gas interface in the tunnel is an important premise to ensure the normal operation of the tunnel. Based on the characteristics of distributed temperature sensing (DTS) technology, such as time-space continuity, monitoring convenience and high altitude resolution, a distributed monitoring method of liquid-gas interface and an empirical formula of liquid level height based on DTS technology were proposed, and the applicability of the method was verified by indoor models. The effects of liquid level height, optical cable laying Angle, heating power and optical cable type on sensing performance are analyzed. The results show that the DTS distributed monitoring method has the best effect when combined with the heating sensor cable in the copper network. When the laying Angle is 45°, the heating time is 2 minutes, and the interface height is 28cm or above, the liquid gas interface height in the tunnel can be accurately reflected.