In order to investigate the complex flow field in counter-rotating turbine and improve effectively the gas turbine design.A numerical study of unsteady flow in a vaneless counter-rotating turbine was presented in this paper.The steady and unsteady results were contrastively analyzed.It's found that the unsteady effect can decrease the specific work ratio appreciably,and increase the loss by about 0.44% efficiency points.However,the mass flow remains unchanged.The results show that the unsteady interactions from the wake,shockwave,potential field and the tip leakage flow have made great contributions to the unsteady effect.