Rare-earth thulium (Tm3+) doped yttrium oxide (Y2O3) host single crystals are promising "eye-safe" laser materials. However, the mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals are not yet understood at the fundamental level. Here, we synthetize a series of Y2O3:Tm3+ samples by the sol-gel method. Our experimental results show that the most intensive absorption line of the 3H6 → 1D2 transition occurs at 358 nm, and the strongest emission line of the 1D2 → 3F4 transition is located at 453 nm, which are in good agreement with the calculations of 363 nm and 458 nm, respectively. By using the CALYPSO structural search method, the ground state structure of Y2O3:Tm3+ with P2 space group symmetry is uncovered. The complete energy levels, including free-ion LS terms and crystal-field LSJ multiplet manifolds, of Y2O3:Tm3+ are obtained based on our developed WEPMD method. The present findings show that our WEPMD method can be used in experiments to elucidate the underlying mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals, which offer insights for further understanding of other rare-earth doped laser materials.