Long afterglow materials are widely used, but their luminescence mechanism is difficult to be universally explained. For Sr2MgSi2O7:Eu2+,Dy3+ silicate long afterglow material with good luminescence-afterglow properties, the molecular models of Sr2MgSi2O7 substrate, Eu doped and (Eu,Dy) co-doped Sr2MgSi2O7 were constructed, and the first-principles calculations were carried out. From the perspective of electronic structure, the electron transition trapping path is revealed to explain the persistent luminescence mechanism of Sr2MgSi2O7:Eu2+,Dy3+. The results showed that the doping of Eu and Dy ions changed Sr2MgSi2O7 from an indirect band gap semiconductor to a direct band gap semiconductor. The Dy 5d state was mainly located between the Fermi level and the Eu 5d state, and it had energy overlap with the Eu 5d state, which confirmed the rationality of Dy3+ as an electron trap. The luminescence process of Sr2MgSi2O7:Eu2+,Dy3+ was revealed, which is helpful for the subsequent regulation and improvement of optical properties.