We investigated the effect of trivalent lanthanide substitution on a novel oxynitride persistent phosphor SrSi2AlO2N3:Eu2+,Ln(3+), which shows green persistent luminescence for more than 2 h. First, an energy level diagram by using the host-referred binding energy (HRBE) scheme was constructed. The location of the energy levels of all divalent and trivalent lanthanides referred to the energy band of the host SrSi2AlO2N3 was estimated. Then, thermoluminescence (TL) measurements in the target persistent phosphors were performed to obtain direct experimental results on the trap depth. We found that the trap levels based on the TL measurements coincided well with the 4f ground states of divalent lanthanide codopants in SrSi2AlO2N3:Eu2+,Ln(3+). The strongly suggests the effective traps for persistent luminescence in SrSi2AlO2N3:Eu2+,Ln(3+) could be due to the aliovalent substitution of Ln(3+) for Sr2+, which can be controlled by selecting suitable codopant Ln(3+). The work shows the HRBE scheme may offer a way to understand the nature of defects in the persistent phosphor as well as a possible guideline to design new persistent phosphors with required trap depths.