The variation rule of Tritium in Lithium deuteride coincides with the naturial Tritium decay incompletely. For determing its rule, the follow-up measurement of the guantity of Tritium in storing Lithium deuteride-tritide has been done for a long time. Using measurement data and polynomial regression and conbining with Tritium natural decay rule,this paper establishes a variation mathematic model of Tritium in Lithium deuteride-tritide. The model has very small forcasting errors at the measuring points.