The present invention provides a backlight module and a support device for a sheet-form optical element contained within the backlight module. The backlight module mainly includes a frame, a support device and the sheet-form optical element disposed within the frame. The support device includes a support portion and a positioning portion. The support portion has a support end and a connection end. The support end extends into the space within the frame, and supports the bottom end of the sheet-form optical element. The positioning portion has a first end, a second end and a bend portion connecting the first end with the second end. The first end connects to the connection end of the support portion, and an included angle exists between the first end and the support portion. The positioning portion of the support device is inset in the frame to provide a support force for the support portion.