Method for automatically adding scale symbol to operation formula during operation and system for executing the method. The system includes: an input unit for keying in operands and choosing the scale thereof; an input/output buffering unit for temporarily storing the input data; a synchronously checking unit for checking the whether the input operands conform with the scale; a scale symbol adding unit for reading and automatically providing the symbol of the scale and sending the symbol to the input/output buffering unit and a mathematic logic calculating unit to be operated and wait for further instruction. After operated, the data are displayed by an output unit or stored in a memory. The data can be stored in both a read-only memory for check in next use or temporarily stored in a dynamic random access memory for checking the data source of the result and the scale.