Anaerobic digestion (AD) is an effective method to treat swine manure and recover energy. However, swine manure with high total solid concentration often leads to long startup time of anaerobic digesters, low degradation efficiency of organic matter and incomplete fermentation. Herein, the hydrothermal hydrolysates of digestate obtained at two centrifugal speeds (i.e., the supernatant derived from 4000 to 10,000 rpm, respectively, named H4000 and H1000) co-digested with swine manure were conducted to investigate the effects of hydrolysate addition on AD startup and performance. Although the concentrations of organics were a slightly higher in H4000 than in H10000, a rapider biogas production occurred in the co-digestion of the H10000 hydrolysate and swine manure, indicating that the finer hydrochar was more optimal to prompt AD startup and performance than larger hydrochar. The fine hydrochar in the hydrolysate had a higher charge storage capability, and lower electron-transfer resistance, which could enhance the direct interspecies electron transfer between bacteria and methanogens and microbial activity. Also, hydrolysate addition could promote the growth of potential exoelectrogenic bacteria in AD. These findings provide a deep understanding of the effects of hydrolysate on the AD systems and are helpful for developing AD techniques for swine manure treatment.