In this study, the treatment performance of dairy farm liquid digestate by freshwater microalga Chlorella vulgaris NIES-227 was investigated and the potential of biomass utilization was evaluated. In column photobioreactors, Chlorella vulgaris was used to treat wastewaters containing unsterilized liquid digestate at concentrations of 25%, 50%, 75%, and 100%. The results showed that the removal of total nitrogen (TN), total phosphorus (TP) and COD were 36.0%—92.5%, 42.8%—100% and 6.9%—32.2%, respectively. The microalgae exhibited the maximum removal efficiencies of nitrogen and phosphorus in 25% liquid digestate, and the maximum removals of ammonia nitrogen (NH3-N), TN and TP were 99.9%, 91.0% and 100%, respectively. The microalgae grew well in low concentrations of liquid digestate (25%—50%), and the highest biomass productivity was obtained in 50% liquid digestate with the value of 393.6 mg/(L·d). However, microalgal growth was inhibited in higher concentrations of liquid digestate (75%—100%), which led to the decreases of the removal of nitrogen and phosphorus. The number of bacteria increased significantly during the cultivation among all the treatment groups, which was beneficial to the COD removal. The contents of total lipid, total sugar and protein in biomass harvested from different concentrations of liquid digestate were 13.2%—32.2%, 12.3%—27.6% and 16.2%—30.9%, respectively. The experimental data show that low-concentration biogas slurry can produce more high-energy components of biomass, which is suitable for biofuel development; high-concentration biogas slurry can produce biomass with more protein, which is more suitable for animal feed.