Good performance of low-density ceramic proppant is particularly important during the exploration of unconventional energy. In view of the performance degradation of low-density ceramic proppant upon serving in the formation water, two specifications of proppant were selected to be soaked in the formation water for different time. The proppant before and after soaking was characterized by XRF, XPS, XRD, SEM and TEM, and its aging mechanism was explored. The results show that the content of ClO2 in the proppant increases, while the content of Al2O3 and SiO2 decreases, and the content of Cl- and HCO-3 in the formation water decreases. The peaks of Cl 2p and Cl 2s appear by XPS, and the new phase SiCl4 is detected by XRD. These results confirm that Cl- in the formation water diffuses to the proppant surface and reacts with SiO2 to form SiCl4, which further hydrates to generate silicone gel or silicic acid gel and coats the proppant surface. Al2O3 in the proppant can react with H+ (acidic environment) of the formation water to form Al3+, and then Al3+ reacts with HCO-3 to form Al(OH)3 precipitating into the formation water. A large amount of Cl- and H+ in the formation water erode the proppant from the surface, reducing the densification within the structure and making the structure loose, which reduces the proppant crushing resistance and degrades its performance.