The utility model provides a cable duct lighting device for detecting hazardous. The device comprises a front lighting chamber (1), a ventilating hole (2), a buzzer (3), a threshold setting shifting sheet (5), a switch (6), an LED lamp strip (7) and a cell chamber (8); a threshold setting module, a comparison circuit module, an LED lamp driving module, a buzzer driving module, a lighting module and a logical OR gate module are arranged in the front lighting chamber (1); a sensor module for detecting the concentration of hazardous gas is arranged in the ventilating hole (2). With the adoption of the device, the problem that a power detector and other non-professional hazardous gas detectors cannot detect the concentration of hazardous and alarm on real time when working in a cable duct is effectively solved; the device is small in size, convenient to carry, simple and reliable, and can be used as a lighting device while detecting hazardous gas.