A calculating device capable of displaying constants and the method thereof are proposed. A key determination unit is provided in a calculator to determine the function of the pressed key of an input unit. A constant unit database is provided for storing constant data. A constant execution unit is provided to read out the content of the constant unit database according to the determination result of the key determination unit. An output unit is also provided to display the value, unit, symbol and item number of a constant. During operation, a user can directly input the item number of a constant in a constant menu frame. Or after a user switches the page of the constant menu frame to find out a desired constant, he can then directly input the constant to display the value, unit, symbol and item number of the constant.