Abstract: With ethylene glycol (EG) as solvent and polyethyleneimine (PEI) as surfactant, upconversion nanoparticles (UCNPs) with polycrystalline shell structure were synthesized via solvothermal method. The polycrystalline shell is confirmed and calculated by HAADF, HRTEM and size distribution from TEM. Shell thickness is readily tuned by control the amount of reactant. When the polycrystalline shell thickness is 1.7 nm, the upconversion luminescence intensity reaches the maximum, which is different from the situation of single crystal shell. The possible reason is that a large number of grain boundaries in the polycrystalline shell lead to a serious loss of energy transfer in the Yb-Yb long distance energy migration.