Red sandstone, as a building material, plays a vital role in the history of Guangdong province. Many red sandstone cultural relics had significant historical, scientific and artistic value are preserved in Dongguan city. Most of the relics are outdoor sites, and therefore had serious deterioration problems due to various causes. Currently, conservation efforts for these cultural relics have not been deployed effectively. A survey of the preservation status of these red sandstone cultural relics was made using scientific analytical methods such as X-ray diffraction (XRD) to study the deterioration mechanism of those red sandstone sites. The results suggest that the main factors causing deterioration of red sandstone relics are water (precipitation, acid rain and capillary water) and soluble salts. This conclusion provides a reliable basis for future projects on conservation of red sandstone cultural relics.