Coordinated multi- point (CoMP) system has been applied to the 4th generation wireless communicationnetwork LTE- A and beyond networks to accommodate ever- growing need for system capacity and diversity.It is par-ticularly suitable for cell- edges.However,there exists bandwidth limit and transmission delay in the non- idealbackhauls of LTE- A CoMP system,which results in imperfect CSI (Channel State Information) feedback from userequipment to base stations and further causes the degradation of CoMP performance.In order to solve this problem,a CSI quantization method on the basis of Grassmann manifold theory is proposed.With this method,an optimalcode book can be obtained to reduce the CSI feedback overhead.Both mathematical analysis and simulation showthat the proposed method helps improve the CoMP performance under non- ideal backhaul constraint.