<p>Earthen plasters are widely used around the world, but their poor water stability has resulted in many deteriorations. Therefore, natural hydraulic lime (NHL), which has superior performance, is mixed with earthen plaster for conservation/restoration work. However, the effect of NHL on the water-retention properties of the plaster has not been fully investigated. For this purpose, several types of NHL blended plasters were prepared, and the effect of NHL on the water-retention properties of earthen plaster was investigated by pressure plate tests, filter paper tests, mercury intrusion porosimetry tests, and nitrogen adsorption tests. The results show that the addition of NHL significantly affects the water-retention properties of the earthen plaster (Dengban clay). Compared with Dengban clay, the NHL blended plasters have higher air-entry values and a steeper water-retention curve in the capillary dominant stage due to the change of pore structure, while in the adsorption dominant stage, the NHL blended plasters adsorb less water due to the decrease in specific surface area and have smaller hysteresis. For the earthen plaster murals in the grottoes of northwestern China where the relative humidity can range from 20 to 85%, the degree of saturation of the Dengban clay can be up to 21.5%. Therefore, a lower saturated blended plasters of NHL2 (16%) and NHL5 (17%) is more suitable for conservation. Considering that Dengban clay can have a degree of saturation up to 21.5%, it is important to eliminate capillary water in the routine maintenance of the earthen plaster.</p>