The co-fermentation of mixed substrates is a promising process compared with single substrate fermentation with regard to substrate characteristics. In this paper, the technical feasibility for improving hydrogen production by co-fermenting sewage sludge and flower waste at different mixed ratios was evaluated using batch tests. The experimental results showed that co-fermentation of flower waste and sludge synergistically improved the efficiency of hydrogen production. The highest hydrogen yield was 39 mL/g-VSadded (VS, volatile solid) when the ratio of sludge to flower waste was 10:90, which was 460% and 30% higher than those from only sludge and only flower waste, respectively. The maximum VS removal was 15.2% when the sludge/flower waste ratio was 10:90, compared with the values of 4.4% and 10.6% from sludge and flower waste, respectively. Co-fermentation induced higher microbial activity and a more efficient fermentation pathway. Furthermore, co-fermentation enriched high relative abundances of both Clostridium sp. (48.1%) and Enterococcus sp. (37.8%) for more efficient hydrogen fermentation.