High-solid anaerobic digestion (HSAD) of horticultural waste by adding alkali (ADAA) was optimized in a batch system, and its performance was compared to co-digestion in a semi-continuous system. In the batch system, the process of ADAA was optimized by testing the time point of adding alkali and the type of alkali, both of which had significant influence on HSAD performance. The cumulative biogas yield of poplar leaf was the highest, 156.7 mL/g of volatile solids (VS), when adding CaO on the third day. In comparison to NaOH and Na2CO3, the biogas yield of the ADAA process adjusted with CaO was the highest. When the organic loading rate was 4.0 and 8.0 g of VS L-1 day(-1), the co-digestion system would produce 10.30 and 15.28% more methane than ADAA (CaO as alkali), respectively, which was due to the higher nutrients, broader diversity, and better growth of the microorganism in co-digestion of poplar leaf and chicken manure. Microbial community analysis of the seed sludge, co-digestion, and ADAA showed Methanosarcina, Methanosaeta, Methanobacterium, Methermicoccus, and Anaerosphaera were found to be the dominant methane -producing bacteria. The study confirms the advantages of co-digestion over ADAA for methane production from horticultural wastes.