A conveyor with a weighing system includes a conveyance unit, a length calculation module, a weighing module, and a controller. The conveyance unit conveys a cargo to move. The length calculation module is arranged at one side of a front end of the conveyance unit to acquire a length of the cargo. The weighing module is arranged at a bottom part of the conveyor. The controller is connected to the length calculation module and the weighing module, such that based on the length acquired by the length calculation module, a weight of the cargo measured by the weighing module is provided. A weighing method of a conveyor is also provided.