An over-long floating-point data type based on the C + + Object Oriented Programming ( OOP) is designed and implemented in this paper. The data type is beyond the limitation of software data type and computer digit length and can accomplish storage and arithmetic operation and input/ output among the over -long floatinglpoint numbers given at discration. On the basis of the data type , we advance a series - based method by which we can calculate values of circurnference ratio π and the natural logarithm's fundus e up to any digit after the radix point.