Since LuTaO4 is a scintillator host with the highest density, it is important to investigate the structure and phase transition for its single crystal preparation. The polycrystalline formed by the mixture of Lu2O3:Ta2O5 with the molar ratio 1:1 is prepared by solid state reaction method at different temperatures. The phase transitions and structures of the polycrystalline powders are investigated by X-ray diffraction and Rietveld refinement. The results show that the polycrystalline has a single phase M'-LuTaO4 when sample is prepared at 1740 ℃, it presents a mixture phase of M'-LuTaO4 and M-LuTaO4 at 1800 ℃, and it displays a single phase M-LuTaO4 at 1840 ℃. The sample is melted when the calcined temperature is 2058 ℃, the melt is quenched and the polycrystalline is the mixture of M-LuTaO4, Lu3TaO7 and Ta2O5, whose structural parameters, including the lattice parameters, atomic fraction coordinates, etc. are obtained by Rietveld refinement to their X-ray diffraction pattern, and the results show their weight ratios are 78.1%, 18.9% and 3.0%, respectively. These results are valuable for the single crystal growth of the heavy scintillators with the host LuTaO4.