The invention discloses an unattended operation automatic weighing system which comprises an industrial personal computer, a wagon balance weighing unit, a gateway control unit, a signal lamp controller, an inlet recognition coil, an outlet recognition coil, a bar code recognition unit, a vehicle license plate recognition unit, an image snapshot unit, an LED display unit, a language prompt unit, a storage server, an inlet signal lamp, an outlet signal lamp, an inlet car stop and an outlet car stop, wherein the wagon balance weighing unit, the gateway control unit, the signal lamp controller, the inlet recognition coil, the outlet recognition coil, the bar code recognition unit, the vehicle license plate recognition unit, the image snapshot unit, the LED display unit, the language prompt unit and the storage server are respectively connected with the industrial personal computer; the inlet signal lamp and the outlet signal lamp are connected with the signal lamp controller; the inlet car stop and the outlet car stop are respectively connected with the gateway control unit. The invention further discloses a weighing method on the basis of the weighting system. By means of scientifically and reasonably specifying and simplifying work procedures, weighing and metering are automatically achieved, weighing data are automatically input into an enterprise ERP system, work intensity of a weighman can be greatly reduced, mistakes occurring during weighing are reduced, the phenomenon that the weighman cheats is reduced, and internal management control of an enterprise is improved.