An insect-trapping device includes a container, a funnel element, two photo interrupters and a controller. The container has an inlet and an accommodation space. The funnel element is placed on the inlet to extend into the accommodation space of the container. The photo interrupters are arranged within an inner passage of the funnel element along a long axis direction of the inner passage for sensing whether at least one insect passes through the inner passage from the inlet. The controller is electrically connected to the photo interrupters for counting the insects passing through the inner passage according to the photo interrupters.