A novel host lattice disodium calcium ditin(IV) trigermanium oxide Na2CaSn2Ge3O12 was utilized for synthesizing long-persistent phosphorescence materials for the first time. Reddish orange long-persistent phosphorescence was observed in Na2CaSn2Ge3O12:Sm3+ phosphors with persistence time more than 4.8 h. The phosphors were synthesized by a conventional solid-state reaction pathway in air atmosphere. A predominant cubic phase of Na2CaSn2Ge3O12 was observed in all XRD patterns. Photoluminescence measurements indicated that the emission spectrum was composed of the peaks located at 566 (the strongest), 605, 664, and 724 nm. The results of the decay curves in terms of a biexponential model suggest that different defects appear in the crystal lattice. The defects acting as traps were investigated by thermoluminescence, which demonstrated that doping Sm3+ ions into the Na2CaSn2Ge3O12 host has made the trap types abundant. Furthermore, the origin of the long-persistent phosphorescence has also been discussed. On the basis of the above results, Sm3+-doped Na2CaSn2Ge3O12 phosphors are considered to have potential practical applications.