A tray mechanism is configured to mount an electronic device on a mainboard device. The mainboard device includes a casing and a mainboard disposed in the casing. A casing side-plate of the casing has a pin. The tray mechanism includes a tray and first and second sliding sheets. The tray has a bottom plate and a tray side-plate corresponding to the casing side-plate. The electronic device is disposed on the bottom plate. The first and second sliding sheets are slidably arranged on the tray side-plate and have first and second oblique slots intersecting with each other. First and second bottom end portions are formed at sides of the first and second oblique slots away from the bottom plate respectively. When the first and second sliding sheets are located at a mounting position, the pin penetrates through the first and second bottom end portions simultaneously.