Better understanding the processes of Cr(Ⅵ) microbial reduction to Cr(Ⅲ) is of great interest in bioremediation of contaminated environments. In this study, the paddy soil suspension was used as microbial inoculation and humic acids were extracted from materials as weathering coal of Datong in Shanxi Province, lignite of Gongxian in Henan Province and Dianchi Lake sediment (abbreviated respectively as HAs, HAh, HAk). The effects of humic acid on microbial Cr(Ⅵ) reduction were studied by adding glucose or humic acid (HA) solely/together in anaerobic aqueous condition. The results indicated that the microbial Cr(Ⅵ) reduction process was promoted by humic acid which acted as electron shuttles in the presence of the available carbon source. The stimulative effect on the microbial Cr(Ⅵ) reduction enhanced with the concentrations of humic acid added, the concentration as 0.02 g/L almost had little effect while 2.00 g/L reduced above 95% of the Cr(Ⅵ) in solution. HA extracted from different materials had distinct influences on microbial Cr(Ⅵ) reduction. HAk and HAh had greater effects than HAh for their higher degree of humification, HAk contained certain polysaccharides or polysaccharide-like which also advanced the processes of microbial Cr(Ⅵ) reduction. The effect of HAk on microbial Cr(Ⅵ) was very weak.