_{: Phosphate adhesive were prepared with the H3PO4 and Al (OH)3 as the matrix and nano CuO as the inorganic fillers. The influence of nano CuO on the water resistance of phosphate adhesives was investigated by soaking mullite joints that were bonded by the adhesive in artificial seawater for 168h. The results show that the nano CuO could improve the water resistance of the adhesive. The shear strength of mullite joint was 7.13MPa which is the best strength after immersed in artificial water whenthe content ofnano CuO was 20wt% in matrix. It has good water resistance within the whole heat-treatment temperature range from 150^oC to 1300^oC. The lowest shear strength is 2.31MPa after heat-treatment at 1300^oC and immersed in artificial seawater. The effects of nano CuO on the water resistance of phosphate adhesive were analyzed by XRD and SEM.}