Process of chloride ion diffusion in concrete with cracks is studied to accurately assess durability performance of concrete structure with cracks. Preparation of cracks with nondestructive method is proposed, which can produce cracks in internal concrete or on its side face effectively. Chlorine salt solution immersion experiments on the concrete with cracks is conducted. It is studied on the chloride ion diffusion effect of specimen with single slit and double slits, and revised chloride ion diffusion coefficient, and simulating chloride ion diffusion process of concrete with cracks. Research results show that the chloride ion diffusion will occur along or perpendicular to the cracking direction, and decreasing of the water cement ratio, resistivity to chloride ion diffusion in cement mortar specimens increased significantly. In a specific range, increasing cracks spacing, the chloride ion diffusion interaction between double slits obviously decreases. And simulation results in ANSYS are in good agreement with those data obtained in test.