A 5-month anaerobic co-digestion was conducted using landfill site closure leachate as the inoculum stock solution and kitchen waste as the digestion material. This study investigated biogas production at varying kitchen waste loading rates, as well as changes in humic substances and microbial diversity in the digestate. The results demonstrated that when the waste loading rate was ≤ 2% (%w/w, dry mass ratio of kitchen waste) , the anaerobic co-digestion system efficiently degraded the kitchen waste, with over 90% of total gas production occurring within one day after the experiment commenced, indicating the rapid completion of the gasification process. The analysis of the digestate from the anaerobic co-digestion showed that the humic substance content was more than 20 times greater than that of landfill site closure leachate within 1 month, but the value approached the landfill site closure leachate within 5 months. This indicated the biological transformation of kitchen waste into humic substances was completed within 1 month, and these humic substances formed was essentially degraded within 5 months. The anaerobic co-digestion accelerated the process of kitchen waste spoilage. Additionally, the bacterial and archaeal communities in landfill site closure leachate and the digestate from the anaerobic co-digestion were analyzed, revealing that dominant bacterial groups were Proteobacteria(29.73% ~ 58.34%), Firmicutes(12.76% ~ 27.18%), and Bacteroidetes(9.07% ~ 28.17%), while the archaeal community included Methanobacterium(2.73% ~ 38.19%), Methaneta(1.05% ~ 40.09%), Methanosarcina(8.94% ~ 58.07%), and Methanoculleus(0.57% ~ 33.19%). These findings indicate that landfill site closure leachate serves as the primary source of methanogenic bacteria, and external sewage inoculum is unnecessary for the anaerobic co-digestion of landfill site closure leachate and kitchen waste. These research results will offer data support and technical support for the sustainable utilization of organic solid waste.