Biofuel production from microalgae biomass via fermentation is promising as a renewable technology. Hydrothermal hydrolysis pretreatment of microalgae slurry prior to fermentation is an efficient method to enhance the biogas production. Herein, we report, for the first time, the hydrothermal hydrolysis pretreatment of microalgae slurries in a continuous reactor under subcritical conditions (temperature < 200 degrees C, pressure similar to 2MPa). The results show that the yield of carbohydrates from microalgae slurries first increased and then decreased with increasing average outlet temperature, which reached a maximum value at 160 degrees C for 10 min. This phenomenon was caused by the combined effects of the release of organics and biochemical reactions. Notably, the radial temperature distribution of microalgae slurries, influenced by the mass fraction and the flow rate during the heating process, had effects on the yield of organics (carbohydrates, proteins) in aqueous solution. This study provides useful information for the operating conditions in large-scale applications.