On basis of cluster supply chain with two four-echelon single supply chains, cross-chain inventory coordination between two retailers is taken into account on condition of the two supply chains co-petition. The time-varying robust control system model is built, the method of quantizing bullwhip effect is put forward, and optimal orders are explored through utilizing Hinfin control arithmetic. The simulation example proves that cross-chain inventory cooperation in cluster supply chains and robust control method can effectively weaken bullwhip effect, reduce stock, dwindle fluctuation of order and improve response to cluster market.