For the efficient utilization of algal biomass for gaseous biofuel production, hydrothermal acid pretreatment was employed in this study to hydrolyze algal bloom biomass harvested from Dianchi Lake in Yunnan Province of China. The hydrolyzed algal biomass was subjected to sequential dark hydrogen fermentation and anaerobic digestion to improve energy conversion efficiency (ECE). The results of X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy indicated that hydrothermal heating with increasing concentrations of sulfuric acid (H2SO4; 0-8%) led to stronger damage to amorphous cellulose, which resulted in the increase in crystallinity index of cellulose. Hydrothermal heating with 4% H2SO4 resulted in the maximum ECE of 55.7%, which yielded 37.33 mL H-2/g.total volatile solids (TVS) and 261.93 mL CH4/g-TVS. However, the yields of hydrogen and methane were significantly reduced after hydrothermal pretreatment with 8% H2SO4 which was most likely due to the substantial inhibitory effects on the bacteria and methanogens, caused by excess sodium ions introduced during pH adjustment process.