Municipal sewage sludge was immobilized with a modified alginate gel entrapment method, and the immobilized cells were used to produce hydrogen gas in a three‐phase fluidized bed. The hydrogen‐producing fluidized beds were operated at different liquid velocity (U0) and hydraulic retention time (HRT). The results show that in response to operating liquid velocities, the fluidized‐bed system had three flow regimes, namely, plug flow, slug flow, and free bubbling. Pressure fluctuation analysis was used to analyze the hydrodynamic properties in this three‐phase fluidized bed when it was under a steady‐state production of biogas. With a steady‐state biogas production rate (Ug) of 0.196 mL/s/L, a transition state occurred at a liquid velocity (U0) of 0.85 cm/s. As U0< 0.85 cm/s, the system was basically a nonhomogeneous fluidized bed, whereas the bed became homogeneous when U0was higher than 0.85 cm/s. The fluidized bed can be stably carried out at high loading rates (HRT as low as 2 h). Hydrogen fermentation results show that the maximal hydrogen production rate was 0.93 L/h/L and the best yield (YH2/sucrose) was 2.67 mol H2/mol sucrose.