Three weirs (Shanhe weir, Wumen weir and Yangtian weir) in Hanzhong city, Shaanxi province was successfully listed in the world irrigation engineering heritage list in October 2017. Among them, Shanhe weir is the earliest irrigation project in Hanzhong region. During the hydrological archeological surveyin the 1980s, it was found that the Shanheyan masonry was made of wooden stake masonry as a skeleton and tri-soil composed of yellow mud, sand, lime, Tung oil and glutinous rice pulpas the filler material. In this study, mortar samples from the east embankment and Yangtou weir of Shanhe weir were detected by X-ray diffraction analysis (XRD), fourier transform infrared spectrometry (FTIR), thermal analysis, scanning electron microscopy, petrographic analysisand 14C dating technology to determine the date, composition and scientific mechanism. The result of 14C dating of the stone strips of the East dike ofShanheweir was that they could date back to 1150 to 1226 AD, which indicated that the date of Shanhe weir could be be date back to the Southern Song Dynasty. Based on the results of XRD and FTIR, the inorganic components of the East dike and rammed earth and Yangtouweir were mainly calcite, quartz and a small amount of feldspar. Infrared analysis(FTIR) and thermal analysis showed that some organic matter appears to be added to the clay and rammed earth. Combined with the literature, the organic matter was likely to be glutinous rice pulp. The heat analysis results showed that the exothermic peak above 800 ℃ was the characteristic peak of calcium silicate hydrate, and it was assumed that lime reacts with water to produce calcium silicate. Calcium silicate can harden better in water or in moist environments. It is the main reason why water Conservancy projects can be preserved for thousands of years. The results of electron microscopy showed that the non-carbonized decalcitrate(Ca(OH)2) in the sample was dehydrated to form a hydroxycalcite crystal. In addition, the organic matter added to the mortar and the slow carbonization of the mortar are helpful to enhance the self-repair ability and weathering ability of the mortar masonry and extend the service life of the water Conservancy project. This work uses various natural science means to carry out scientific research on the ash pulp ofShanhe Weir mortar, which is helpful to find out the composition and scientific mechanism of the mortar material of water Conservancy project, lay a scientific foundation for the protection and repair of water Conservancy project heritage such as Shanhe Weir, and provide useful enlightenment for the improvement and enhancement of modern water Conservancy materials.