In order to develop a new mortar admixture and effectively apply black fluid from paper making, the air entraining, surface activity of the black fluid was studied, and its effect on the workability, water retention, setting time and strength of the mortar was investigated. The results show that the black fluid have good air entraining property and surface activity. When the use level (by mass)of black fluid is 03%, the consistency loss of mortar mixed with the black fluid is 123% after 2h, the setting time is 49h longer than that of the control mortar, and the ratio of water retention rate to that of the control is 1086%. For the hardened mortar, when the black fluid use levle is 03%, the adhesive strength ratio of the mortar mixed with the black fluid is 115% at the age of 14d, the compressive strength is 309MPa at the age of 28d. To meet the requirment set forth in the JG/T 426—2013 (Plasticizer for plastering mortar) standards, the black fluid should be combined with a suitable amount of coagulant, and the proper use level in the mortar is 03%.