Abstract: Based on the basic principles of thermodynamics， the driving conditions for water vapor transfer in concrete and the water loss mechanism of steam-cured concrete were analyzed， and the effect of water loss during steam curing on the mechanical strength and microstructure of concrete was studied. The results indicate that higher steam curing temperatures lead to an increase in the number of internal pores in the concrete， while exacerbating the degree of microstructural damage. The water vapor transfer in concrete depends on the partial pressure of water vapor in the atmosphere （i.e.， the absolute humidity） and is independent of the relative air humidity. The air humidity in the surrounding atmosphere is much lower than that in the concrete pores， which drives the water vapor in concrete to evaporate into the atmosphere and causes rapid water loss. This leads to the shrinkage of the concave liquid surface of pore water in concrete， and the shrinkage tends to connect with internal air bubbles， ultimately leading to the inevitable formation of microcracks. The propagation of water loss-induced microcracks is the primary reason for the reduction in concrete strength caused by steam curing.