To develop a new type of mortar suitable for base surface leveling of primary support with some permeability, poplar sawdust is added into the mortar test block to modify its performance. The degradation characteristics of poplar sawdust are analyzed, and poplar sawdust with different dosages is added into the mortar test block to change its curing mode. Furthermore, the basic performance indices and microstructure, such as volume density, porosity, water absorption, and compressive strength, are tested and analyzed. The results reveal the following: (1) The porosity and water absorption of the mortar test block increase by 2~3 times after the addition of poplar sawdust, but the volume density and compressive strength of the mortar test block decrease. (2) Under the conditions of alkali addition and curing in alkaline solution, porosity and water absorption are the highest, whereas volume density and compressive strength are the lowest. After linear fitting of the water absorption rate and compressive strength of the mortar test block, these factors are found to be negatively correlated. The results show that it is feasible to improve the water permeability of mortar using poplar sawdust, but the influence of poplar sawdust dosage and curing mode on other properties of mortar needs further study.