Fe2+ is essential for the improvement of biogas production and the growth of anaerobic microorganisms; however, it is often excessively added to anaerobic digesters, leading to the inhibition of biogas production. In this study, the dosing frequency of Fe2+ for the mesophilic and thermophilic biogas fermentation of high-solid swine manure was optimized. The most significant enhancing effect (13.44 %-33.22 %) induced by Fe2+ addition was observed with dosing frequency of 400 mg/L for every 5 d, and the maximum efficiency for unit concentration of Fe2+ occurred when the dosing frequency was 400 mg/L for every 15 d. Maximum biogas production was obtained in the group with dosing frequency of 400 mg/L for every 5 d, and biogas production potential was 465.24 mL/(g volatile solid (VS)) based on modified Gompertz predicted model with a maximum rate of 16.72 mL/(gVS d), which was higher than that of the control group (6.78 mL/(gVS d)). In addition, Fe2+ dosing displayed a stimulatory effect on SCOD removal, the SCOD removal with dosing frequency of 400 mg/L for every 5 d were highest in all reactors. The optimum dosing frequency of Fe2+ positively affected the microbial community structure. 16S rRNA gene sequencing revealed that the abundances of Firmicutes and Euryarchaeota were increased, which could enhance the hydrolysis-acidification and methanogenesis process during anaerobic digestion process. (C) 2019 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.