The invention discloses a device for detecting quantity of cigarettes packed in a box. The device comprises a sensor (1), a sensor (2), a controller (5) and an alarm (6). The device for detecting the quantity of the cigarettes packed in the box is characterized in that the sensor (1) and the sensor (2) are arranged above an outlet channel of a cigarette box (4) of a cigarette packaging machine, the controller (5) is arranged in an electrical cabinet of the cigarette packaging machine, and the alarm (6) is arranged at the top of the electrical cabinet of the cigarette packaging machine. During operation of the device, the two sensors carry out real-time detection on the cigarette box, when the quantity of the cigarettes packed in the cigarette box is detected to be not enough, the controller (5) immediately locks the cigarette box with the quantity of the cigarettes packed in the cigarette box of being not enough, when the cigarette box arrives at an outlet of the cigarette packaging machine, the controller (5) sends out a stopping signal to the cigarette packaging machine, and meanwhile, the alarm (6) is driven to give an alarm, so as to remind production personnel to isolate and dispose the cigarette box with the quantity of the cigarettes packed in the cigarette box of being not enough.