The invention provides a cable-trench illuminating device capable of detecting hazardous gas. The device comprises a front illuminating bin (1), an air hole (2), a buzzer (3), a threshold setting shifting sheet (5), a switch (6), an LED lamp strip (7) and a battery bin (8). A threshold setting module, a comparing circuit module, an LED lamp driving module, a buzzer driving module, an illuminating module and a logical OR gate module are arranged in the front illuminating bin (1). A sensor module is arranged inside the air hole (2) and used for detecting the concentration of the hazardous gas. By means of the cable-trench illuminating device, the problems that when non-professional hazardous gas detecting persons of the electric power industry and the like enter cable trenches to do work, the concentration of the hazardous gas can not be detected in real time, and alarming can not be carried out are effectively solved. The device is small in size, convenient to carry, simple and reliable, and can be used as an illuminating device while detecting the hazardous gas.