The M2MgSi2O7: Eu2+, Dy3+ (M: Sr, Ca) long afterglow phosphors are investigated by substituting the Ho3+ ions for the Dy3+ ions. The emission intensity is about 3 times and 30% enhanced with this substitution in the Sr2MgSi2O7 and Ca2MgSi2O7 host, respectively. The emissions of the europium ions and the holmium ions are observed. Both emissions attribute to the 5d → 4f transition, indicating that some of the Ho3+ ions are reduced to the Ho2+ ions. The Ho3+ ions or the Dy3+ ions aggregate with the cation and the oxygen vacancies forming the traps. The traps induced by the clusters with Ho3+ ions are shallower than those induced by the clusters with Dy3+ ions. The traps’ concentration of the Ho3+ cluster is lower. The probability of retrapping of the released carriers decreases with the Ho3+ ions co-doping. The combination effect of the shallow traps and the non-retrapping process gives rise to a greater intensity of the initial emission and a shorter duration of the afterglow.