Abstract: The durability degradation caused by calcium leaching is very common in the hydraulic concrete structures. In order to understand the calcium leaching process of the cement paste under the attack of the environmental water, at first, a transport model for calcium ion in the cement paste slice specimen was established by utilization of Fick's laws, the mass conservation law, the solid-liquid equilibrium relationships between calcium ion concentration in pore solution and calcium content in the solid skeleton of cement paste, as well as the Newton law on the boundary of slice specimen immersed into the environmental water. And then, an accelerated calcium leaching experiment of cement paste slice specimens with different water-cement ratios immersed into 6M NH₄Cl solution was carried out to measure the Ca/Si and average porosity of the slice specimens at different leaching time, and the Ca/Si and average porosity calculated by the established model were further compared with that from the accelerated experiments to verify the proposed model. Finally, numerical simulations were performed to analyze the space-time changes of the calcium ion concentration in the pore solution, the solid calcium content in skeleton and the porosity in the cement paste slice specimen. Simulated results show that the results calculated by the model are essentially in agreement with the experimental results; in the early stages of the calcium leaching, the solid calcium content in the cement paste specimens decreases fast, and its porosity has also a rapid increase, but in the later period of leaching, the leaching speed of solid calcium and the increase rate of the porosity gradually decrease.