A　neural　network　on-line　modeling　and　controlling　method　(NNOMC)　is　proposed　in　this　paper　for　multi-variable　control　of　wastewater　treatment　processes　(WWTPs).　According　to　the　approximating　character　of　the　feedforward　neural　network　(FNN),　a　modeling　FNN　is　proposed　to　simulate　and　decouple　WWTPs.　Then,　an　FNN　controller　for　multi-variable　control　of　WWTPs　is　designed.　Moreover,　the　stability　of　the　NNOMC　method　is　proven　in　a　general　inference　via　the　limitation　of　the　learning　rate　of　the　FNN.　Finally,　this　proposed　NNOMC　method　is　used　in　the　international　benchmark　of　WWTPs.　The　results　show　the　NNOMC　method　owns　both　better　approximating　and　controlling　performance.