Eddies can retain water for several months to over a year, and their water properties can change as they move. This study quantifies the change rate of eddies' water properties using potential spicity-density analysis. It reveals that water properties in eddy cores are more conserved than at the edges, with higher change rates during the generation and extinction periods, and decreasing with depth. The change rate is dominated by isopycnal processes in the upper ocean and is higher when interacting with currents. Further statistics and simulations reveal that the water/particles in anticyclonic eddies tend to leave eddies on the left/right side in the North/South Hemisphere. And so, larger change rates can also be found in the leakage sides of the eddies. This study offers a new perspective on eddy evolution and may be crucial for investigating eddy-flow interaction.