In this paper,earthquake disaster degree is divided into five classes:catastrophe,major disaster,medium disaster,small disaster and micro disaster,in which the lower limit Di of disaster magnitude of each disaster class is given. The death toll of earthquake disaster and the relative value of direct economic loss are selected as two indicators to calculate the disaster magnitude. By "(direct economic loss)/(annual per capita GDP)" processing method we adopted the dimensionless direct economic loss,in order to lay a foundation for the comparison of earthquake disaster severity in different years and regions. The elliptic equation is used as the boundary between adjacent disaster classes,and the corresponding parameters are given to determine the expression of the boundary equation. On this basis,strict mathematical derivation is carried out,and a reasonable calculation formula of seismic disaster magnitude model is given. The scientific applicability and comparability of the seismic disaster magnitude model given in this paper,which are verified by hypothetical values and some actual seismic disaster examples in China. The calculation model of earthquake disaster magnitude can be extended to other disasters. By comparing the calculated disaster magnitude values of Tangshan earthquake and Wenchuan earthquake,we find that the disaster magnitude of Tangshan earthquake is greater than that of Wenchuan earthquake,and the rationality of this result is discussed.