Corn stalk pith, a low-cost organic material, was first utilized to produce hydrogen by photo-fermentation in a long tubular photobioreactor (PBR) with continuous mode. The effect of hydraulic retention time (HRT: 12, 18, 24, 30 and 36 h) on hydrogen production and light conversion in the tubular PBR was evaluated. The distributions of cell optical density, pH, sugar and soluble metabolites products (SMPs) along the tubular PBR (0 cm, 25 cm, 50 cm, 75 cm, and 100 cm from the inlet) were also assessed under different HRTs. The HRT of 24 h was found to be the optimum condition with the highest hydrogen production (2670 mL), yield (211.9 mL/L-medium), hydrogen production rate (38.4 mL/L/h), and light conversion efficiency (0.42%). In addition, statistical analysis revealed that cell optical density, pH value, sugar concentration and SMPs varied with different locations of the tubular PBR. Moreover, HRT of 24 h and 36 h accounted for 2.6-1.3% and 4.0-11.8% of the total SMPs in the effluent, respectively. Besides, acetic acid and butyric acid were the main SMPs in the effluent. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.