The on-line load balancing is one of the most important problems in the field of algorithm design.Taking into account the practical application scenarios, there are not always required to assign the task immediately when it arrives at the system, we propose a new model of the on-line load balancing problem with an unlimited task buffer.In order to design an efficient algorithm for solving this problem,we introduce the concept of flatness and the corresponding planarization algorithms in this work, and then propose three algorithms based on the main idea of LPT algorithm and planarization method,named as LPTCP1, LPTCP2 and LPTCP3, to solve this problem.Experimental results show that our algorithms can effectively solve the on-line load balancing problem with task buffer and have good performance in the large size problem.