A backlight module, an assembly method of the backlight module, and a display device are provided in the disclosure. The backlight module includes a backplane, a light bar, a light-guide plate, a reflective sheet fixed on a first side of the backplane, and an adhesive layer. The light bar includes a circuit board and multiple light sources. The circuit board has a first region and a second region connected with the first region. The multiple light sources are fixed on the first region. The adhesive layer is fixed on the second region. The light-guide plate is at least partially fixed on the second region via the adhesive layer. The light bar and the light-guide plate are both mounted on the first side of the backplane. The reflective sheet is between the backplane and the light-guide plate.