Cux Zn1-xO were synthesized via the solid-state reaction route. Ferromagnetism was detected when the Cu percentage was bigger than 3%. The compounds were found to be the N-type semiconductors with a carrier concentration of 1015 cm-3. The DFT+U method was employed to calculate the magnetic exchange coupling of the Cu2+O2-Cu2+, Cu2+VoCu2+, Cu2+Vo+Cu2+, Cu2+Vo++Cu2+ in the CuZnO system, where Vo denoted the vacancy of oxygen. Different bound charge transfer schemes between the Vo and Cu2+ ions were revealed. The origin of the ferromagnetism was determined within the framework of the Cu2+Vo++Cu2+ bound magnetic polarons.