An inserting device includes a feeding mechanism, a positioning mechanism, and an assembling mechanism. The feeding mechanism includes a first supporting plate, a second supporting plate, a lifting assembly and a single-axis sliding table. The second supporting plate includes a first place position and an adjacent second place position. The second supporting plate and the first supporting plate can form a receiving space. The lifting assembly lifts trays located on the first place position and the second place position. The single-axis sliding table is received in the receiving space. The single-axis sliding table includes a sliding block. The sliding block receives the tray dropped from the first placing position and further carries the tray away from the first placing position. The positioning mechanism includes a clamping jaw. The clamping jaw clamps a workpiece from the tray and positions the workpiece again. The assembling mechanism assemblies the workpiece on a product.