Abstract: Two kinds of test method for water retention of mortar are analyzed in existing standards, test results of different water retention and thickending materials of mortar and high water retention of special mortar are compared. The results show that the test results increase with tryout filter paper method under the same materals and tryout filter paper method can meet standard requirments easily, however, vaccum take-off method can meet standard requirments hardly. There are significant differences of mortar retentivity for masonry mortar and mortar for plaster and between vaccum take-off method and tryout filter paper method. Vaccum take-off method can effectively distinguish the differences of water retention properties for special mortar. According to different water retention rate, the grade of ready-mixed mortar is proposed. Based on different absorption of wall, the users can choose different water retention of mortar.