An annular optical-fiber-illuminating biofilm reactor (AOFBR) was developed with photosynthetic bacteria (PSB) attached on the surface of side-glowing optical fiber for the formation of biofilm and continuous photo-hydrogen production. To enhance mass transport and improve substrate bio-degradation efficiency within AFOBR, a two-dimensional mass transfer model was proposed based on the theory of mass transfer and kinetics of bio-chemical reaction. Results on numerical investigation revealed that characters of mass transfer obviously affected biodegradation within AFOBR. With the increase of inlet substrate concentration, substrate degradation efficiency was increased at early stage but decreased later. While, with the increase of flow rate, substrate bio-degradation efficiency was always decreased. The maximum substrate bio-degradation of 43.5% was available with 10g/L of inlet substrate concentration and 100mL/h of flow rate. To regulate mass transfer and maintain appropriate substrate concentration within biofilm was proved to be the effect way to achieve higher substrate degradation efficiency within the bioreactor.