The present invention relates to a composite microorganism reactor, wherein both dephosphorization and denitrification occur in one reactor, and an apparatus and a method for water treatment using the same. The composite microorganism reactor according to the present invention comprises: an inner space for accommodating sewage and waste; a partition wall placed in the inner space to divide the same into an anaerobic region for dephosphorization and an anoxic region for denitrification; a sewage and waste inlet portion placed at the upper portion of the anaerobic region; a sewage and waste outlet portion placed at the upper portion of the anoxic region to discharge sewage and waste outside; a sludge discharge hole placed at the lower portion of the inner space to discharge sludge precipitated in the sewage and waste flowing into the inner space; and a sewage and waste agitation device provided at the anaerobic region to agitate the sewage and waste of the anaerobic region, wherein the sewage and waste flowing into the anaerobic region through the sewage and waste inlet portion pass through the lower end of the partition wall to flow into the anoxic region, and then rise at the anoxic region to be discharged outside through the sewage and waste outlet portion.