Abstract: A paper tray includes a casing, a lifting mechanism, a weight detector, a storage unit, and a control unit. An accommodating space is formed inside the casing. The lifting mechanism is disposed inside the accommodating space for supporting and lifting at least one sheet of paper. The weight detector is disposed on the lifting mechanism for detecting a weight of the at least one sheet of paper. The storage unit is for storing paper information and a predetermined amount. The control unit is electrically connected to the weight detector and the storage unit. The control unit determines an amount of the at least one sheet of paper according to relation of the weight and the paper information and outputs a supplementary alert to an apparatus for reminding users when the control unit determines the amount of the at least one sheet of paper is less than the predetermined amount.