Pouring fresh mortar on the substrate is a common repair and reinforcement measure. The water exchange occurs when the moisture degree of the new mortar and the old mortar is different, thus changing the mixing ratio of the repair material, and affecting the hydration process, microscopic characteristics, permeability and bonding performance between the repair material and the old mortar. In this paper, the water exchange process among fresh ordinary mortar, strain hardening cementitious composites (SHCC), and old mortar with different moisture degrees (i.e., 0%, 30%, 70% and 100%) was investigated by a weighing method. The influences of the type of fresh repair mortar and the initial moisture degree of the old mortar on the rate of water exchange and the final water-binder ratio of the repair mortar were analyzed. The effect of the water exchange on the bond strength between the repair mortar and the old mortar, the microstructure of the repair mortar and the water absorption properties was investigated via the mechanical tests, low field nuclear magnetic resonance and weighing method. The results show that there is a good linear relationship between the mass of water absorbed per unit area of the unsaturated matrix and the square root of time in the early stage of water exchange. The shear strength between SHCC or ordinary mortar and substrate is maximum at 0% moisture degree of the substrate. The shear strength between SHCC as a repair material and the substrate is greater than that of ordinary mortar. The porosity, micropores volume fraction and capillary absorption coefficient of ordinary mortar gradually increase and the volume fraction of mesopores and large voids gradually decrease as the moisture of old mortar increases. The volume fraction of SHCC porosity, mesopores and large voids gradually decrease as the moisture degree of old mortar increases, and the volume fraction of micropores and capillary absorption coefficient change slightly.