In order to reduce the operation cost of ammonia nitrogen and phosphorus removal from biogas slurry (BS) through the “thermal-alkaline stripping” process, biomass ash (BA) addition was put forward to elevate the value of pH and precipitate the phosphorus in BS. Totally four types of BA were added into BS and pure water, including rice straw ash (RS), groundnut shell ash (GDS), cotton straw ash (CTS) and corn straw ash (CNS). Main water qualities of those solutions, including chemical oxygen demand (COD), concentration of suspended solid (SS), total phosphorus (TP) and total ammonia content (TAN), pH values, as well as calcium (Ca) and magnesium (Mg) contents were tested. The germination index (GI) values of Chinese cabbage seeds treated by these biogas slurries after CO2 saturated were evaluated as well. Among those BAs, CTS had the highest solubility of about 12.97% and GDS had the lowest value of 1067%. The results showed that when the BAs were added into the pure water, the pH value of water can reach 10.0~12.5. But when the same dosages of BAs were added into BS, the pH value of BS can only be elevated to 9.5~11.0 from 7.87 due to the good buffering characteristics of BS. The main reason of pH value improvement of BS might be the precipitation of CO2 in BS by Ca2+ or Mg2+ leached from BA. This relatively high pH value could meet the requirement of “thermal-alkaline stripping” process for ammonia nitrogen removal, which may contribute to reducing the operation cost greatly. On the other hand, phosphorus removal efficiency was generally increased with the increase of BA dosage. The highest phosphorus removal efficiency of 78.74% was acquired when 100g/L CTS was added into BS, but the efficiency was decreased inversely when BA dosage was increased to 200g/L. The results also showed that when the BA dosage was increased to 200g/L, the removal efficiency of COD and SS were above 50%. For the higher value of pH and phosphorus removal efficiency obtained by CTS addition, CTS was considered as the optimal type of BA. The germination index of Chinese cabbage seeds treated by CO2-rich BS solutions after BA addition was generally higher than 0.8, implying the low phytotoxicity. Therefore, the addition of BA into BS may be beneficial to ammonia nitrogen and phosphorus removal, and may not result in a rapid increase in phytotoxicity.