The water retention capacities of commonly used roof greening matrices, concluding ceramsite, peat and super absorbent polymer, under fully saturated, cyclic wetting-drying and natural evaporation conditions were studied. The saturated water retention capacity of matrices were obtained by vacuum saturated test. It showed that the fully measured saturated degree of saturation of mixed matrix was greater than that calculated by the combination of all components. The reason which increases the water contents to porous skeleton was analysed. In cyclic wetting-drying tests, the water retention capacities of ceramsite and peat were almost unchanged. However, due to the main molecular chain broken under drying in relatively high temperature, the water retention capacity decreased rapidly after several wetting-drying cycles. In natural evaporation tests, the evaporation rates of ceramsite, peat, super absorbent polymer and free water were almost identical in the condition of high water content. As the water content decreasing, the evaporation rate of the matrix which had lower water retention capacities decreases faster. The matrix with a reasonable proportion of super absorbent polymer can remarkably improved its water retention capacity.