Prestack depth migration is computationally intensive. To deal with this problem, people have made big efforts to develop high efficient parallel algorithms. In this paper, after some parallel algorithms are analyzed, according to the character of 3-D Kirchhoff depth migration, a simplified algorithm is presented based on shared memory . In the proposed algorithm, the slave processes store and read ray traveltimes directly so that the total amount of message passing will be remarkably decreased. At the same time, this algorithm has well combined both the "task pool" and the "coarse granularity" techniques together, the former ensuring the dynamic load-balance and the latter reduing the cost of communication between the slaves and the master. In the end, experimental results show that this algorithm has high efficiency and scalability.