Abstract: Ultrasonic assisted method is an effective method to prepare nanomaterials, which can improve their magnetic and optical properties. Described in this paper, GdPO₄:Ce, Tb ternary system fluorescent powder was synthesized under different pH conditions by ultrasonic-assisted. Optical and structural properties of the fluorescent powder were determined by X-ray Powder Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Photoluminesce (PL).The following conclusions are drawn:(1) When pH=1, synthetic samples are 15 nm×104 nm hexagonalnanobars like orthophosphate, green phosphor. Cystals prefer growing at the face of (102) and (200). When pH < 1, no samples can be synthesized, whereas granular orthophosphate green phosphors were synthesized when pH>1. (2) A possible formation mechanism has been put forward, which provided further evidence for the fluorescent efficiency enhancement. Because the 5d energy states of Ce³⁺ ions are high, they can be transferred to the Gd³⁺ ions efficiently. However, the emission transition peak of Gd³⁺ ions and the absorption peak of Tb³⁺ ions have large spectral overlap in the Gd, Ce, Tb ternary system. Therefore, the sequence of energy transmission is the Ce³⁺→Gd³⁺→Tb³⁺. A few Ce³⁺ ions doping to GdPO₄ matrix can enhance fluorescence intensity of the luminescent materials. The result provides theoretical basis for the exploit of rare earth materials.