Abstract: It is always regulated recirculation ratio ( namely the ratio of circulation flow to influent flow in circulation compartment section) by increasing flashboard in circulation compartment section to regulate mixed liquor refluxing from aerobic zone to anoxic zone in modified oxidation ditch． It is studied that the differences of carbon conversion pathways before and after increasing flashboard in circulation compartment section． The results show that by increasing flashboard in circulation compartment section the abilities of hydrolytic acidification and denitrifying phosphorus removal are strengthed， as to promote the use of carbon for nitrogen and phosphorus removal． As a result，the pathway of carbon conversion is changed with a apparent raise of nitrogen and phosphorus removal in modified oxidation ditch．