In order to structure a better continuous flow bioreactor mode for fermentative hydrogen production，a continuous stirred-tank reactor (CSTR)and an anaerobic contact digester (ACR)were evaluated with diluted molasses as raw material，and their operation characteristics during starting and running were analyzed and compared.Inoculated with aerobic pretreated sewage sludge，both the reactors started up at (35±1)℃ with an initial volatile suspended sludge (MLVSS)of 4.8 g·L-1，a hydraulic retention time (HRT)of 12 h and an influent chemical oxygen demand (COD)of 5000 mg·L-1.The pH was controlled at 5.5—6.0 in both fermentation systems during the whole operation period.The results showed that the CSTR could reach steady butyric acid type fermentation in 15 d，much faster than in 25 d for ACR.During the steady state，the butyrate was dominated among the soluble metabolites and counted for 53% and 50% of the total liquid products in CSTR and ACR system，respectively.The hydrogen-producing efficiency was higher for ACR than for CSTR，indicating its better ability to maintain biomass.The substrate acidification ratio，the hydrogen production rate and specific hydrogen production rate of the sludge were 44%，9 L·d-1 and 0.15 L·(g MLVSS·d)-1，respectively，for the steady ACR system，and 1.62，2.05 and 1.15 times of corresponding values of CSTR system.