The non-airtight digestion technology is emerging to be applied in the acidogenic phase for two-stage methane production. However, in this study, it was used to pretreat screened dairy manure (SDM) in order to provide microalgae cultivation with a substrate that might be more suitable for nutrient reduction, especially phosphorus. SDM was firstly underwent non-airtight digestion applying different dilution folds, i.e., blank (no dilution), 5-fold, 10-fold, and 15-fold. Total solids (TS), total dissolved solids (TDS), and chemical oxygen demand (COD) of the SDM were mostly reduced when there was no dilution applied. Five-fold dilution is the most beneficial one for ammonia reduction. Total phosphorus (TP) was reduced the most efficiently in the blank SDM. After the non-airtight digestion, 5-fold diluted original SDM, 5-fold diluted digested original SDM, and digested 5-fold diluted SDM were used to grow microalgae for 8 days. Microalgae grown in 5-fold diluted digested original SDM and digested 5-fold diluted SDM had better removal efficiencies in COD and NH4-N. From the monitoring of pH and TP during the 8-day culture period, it is found that pHs were peaked on the 4th day for microalgae grown in 5-fold diluted digested original SDM and digested 5-fold diluted SDM, corresponding to the maximal TP removal. Non-airtight digestion of SDM could help achieve better nutrient removal by microalgal cultivation in a shorter time span.