The invention provides a long-distance distributed method for monitoring water level depth of a tunnel water drainage channel. According to the method, a carbon fiber internal heating optical cable is arranged; the carbon fiber internal heating optical cable is fixed on a side wall of the water drainage channel through a clamp to form a loop and the loop is communicated with a distributed optical fiber temperature demodulator; the distributed optical fiber temperature demodulator is communicated with a computer; a temperature difference value formed by different temperature gradients or different medium interfaces is measured by utilizing different degrees of energy transmitting in different media of water and air and different temperature gradients formed on the different medium interfaces after heating the carbon fiber of the carbon fiber internal heating optical cable, so that data is demodulated by the distributed optical fiber temperature demodulator and a computer to judge the height of a water level. The long-distance distributed method provided by the invention has the advantages of distributed monitoring, simple structure, convenience for construction, low cost and the like.