Multi buoy moorings （MBM） are commonly adopted in offshore mooring for large oil tankers berthing in open seas，but there are no practical applications in China. We carry out the force analysis of anchor chains which are main structural components of an MBM system，analyze and calculate the force of single chain and composite chains under free or constrained condition by catenary equation，summarize a set of systematic analysis method for the force of chains，and apply the method to an MBM project in the Middle East for case calculation. The results show that the proposed theoretical formula and design method can provide references for the anchor chain design of MBM berth.