To explore the effect of flax cell ulose fiber on the humidity and basic properties of cement mortar,this paper adopted flax cellulose fibers with different lengths (0.5 and 5 mm ) and mixing amounts (mass fraction of 0.5%,1%,1.5%,and 2% of cement ).Tests were conducted on rheological properties,mechanical properties,autogenous shrinkage properties,and internal relative humidity development of cement mortar.Setting time,compressive strength,shrinkage rate,and relative humidity were used to evaluate the performance improvement effect of cellulose fiber on cement mortar.The results show that the absorbance rate of flax fiber per unit mass of 0.5 mm is 3.45 times its weight,while the absorbance rate of flax fiber per unit mass of 5 mm is 6.03 times its weight.Flax fiber has significant advantages in humidity compensation,shrinkage inhibition,and crack retardation of cement-based materials.At each curing age,a higher mixing amount of fiber indicates lower compressive strength.Under the same mixing amount of fiber,the compressive strength of the short fiber group is higher than that of the long fiber group.Compared with the baseline mortar group,the short fiber has a more obvious inhibition effect on autogenous shrinkage.A higher mixing amount of flax fiber indicates more significant internal humidity compensation.In addition,under the same mixing amount of fiber,the humidity compensation effect of long fiber is better than that of short fiber,indicating that the water quantity of internal curing is the key to compensating for the internal humidity.