Abstract: A numercial code for 3-dimensional fluid flow around swimming bodies were developed . The Immersed Boundary method (IBM) was applied to characterize the moving and deformable fluid-body interface in terms of a momentum forcing so that bodies′ motion and fluid flow were uncoupled. The fluid flow was solved by Lattice Boltzmann method (LBM) which was well suited to be parallelized and the body motion was devided into beating which was pre-specified and swimming which was calculatied according to the resultant forces and moments. Then, the self-propulsion of a biomimetic unmanned underwater vehicle (UUV) with multiple fins was simulated, and the effects of fins′ properties and of beating parameters on the propulsion performance were discussed in detail. It was shown that the lateral force generated by symmetric double fins almost vanished, and the trust was enhanced where the vehicle cruise speed was larger than twice the speed by single fin. UUV propelled by flexible fins swam much faster than that by rigid ones. Under the condition of present work, the optimized wavelength of the flexible fin is about 1.25 times the fin length, and the vehicle swam faster with lager beating amplitude or with higher frequency. The propulsion performance of Semi-oval-shaped caudal fin was better than that of deeply forked tail fin when the vehicle paraded with low speed.