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A luminescence kinetic model for the green-emission long-afterglow phosphor (Sr0.5 Ca1.5)MgSi2 O7 :Eu2+,Dy3+ is proposed based on the studies of the thermoluminescence and isothermal decay curves at different temperatures.The isothermal decay curves at different temperatures meet the hyperbolic law and show to be dependent on temperature. Combined with the decay curves and the thermoluminescence curves, it can be concluded that the long afterglow of this material originates from the traps with energy distribution rather than a single level trap. Upon illumination with ultra-violet or visible light, the trap filling can proceed via excitation of 4f electrons to the 5d level of Eu2+ ions. After excitation, 5d electrons can be transferred to the trap related to Dy3+ ions, and Eu3+ is left behind. Upon subsequent heating, the electron is released from this trap and recombines with Eu3+ to produce Eu2+ emission.