Buffer allocation is a major optimization problem in manufacturing systems design. The recursive decomposition model and its statistic characteristics are studied in the unreliable successive product firstly, and the Monte-Carlo simulation method is used to estimate the throughput of product line in the condition of fixed buffer allocation. Comparing with the conventional estimation method, it is easy to depict the production conditions. Then, the tabu set based improved degraded ceiling algorithm is proposed to solve the optional buffer allocation problem. Because of the tabu set with memorability, the search efficient of the algorithm can be increased greatly. Finally, the computer simulation results show that the proposed algorithm can search for the optimum solution effectively and fleetly.