Abstract: Abstract: Tidal power is pollution-free renewable energy and an effective way to reduce coal consumption and guarantee normal social electricity consumption in China. Bidirectional bulb turbine is widely used in tidal power station. Scholars at home and abroad have carried out extensive researches in bidirectional tubular turbine in terms of internal blade clearance flow, the occurrence and location of cavitation and blade airfoil optimization .Scholars in China firstly put forward improving the operating efficiency of the bidirectional tubular turbine under the reverse working condition by means of setting rear guide vanes. In order to solve the low efficiency problem under the reverse condition in bidirectional bulb turbine, how blade tail affected the units' hydraulic performance under reverse working condition by studying bade tails of different shapes and different thickness on the basis of a bidirectional bulb turbine unit with rear vanes was analyzed. The profile of computational domain was built with modeling software UG and the grid division was done with software ICEM, also the grid independence verification for runner parts was conducted. Based on the CFX software, the SST turbulence model was adopted to conduct numerical simulation on runners with blade tails of different shapes and thickness. Results showed, under reverse conditions, the efficiency of the units with circular blade tail was 59.55%, which was higher than 58.4% and 58.01% of units with rectangular and arched tailing edge respectively, indicating that the impact loss of runner with rectangular tail and curved tail was bigger. Moreover, increasing the thickness of the blade tail could improve the efficiency of the turbine relatively obvious under the reverse condition, and the efficiency could reach at 79%, but the method no longer worked when the blade tail thickness increased to a certain degree. The increase of the tail thickness changed pressure distribution on the blade suction surface, and the minimum pressure value reduced by 1 200 000 Pa under reverse condition on the blade tail, which greatly affected the cavitation performance under the reverse condition. Increasing the thickness of blade tail caused squeezing action to the passage channel, which increased the positive condition efficiency but the effect was not obvious. However, the increase of blade's tail thickness decreased the frequency of the Karman vortex at the tail outlet under positive conditions, as the frequency of the unit components was usually low frequency, the probability of the occurrence of Karman vortices on positive condition was increased. The results showed that compared with rectangular blade tail and arched blade tail, the unit with circular blade tail performed the best under reversal conditions. We can conclude that in order to improve the units efficiency under the reverse condition, thickness of blade tail can be appropriately increased. But that could been donein the premise of ensuring Karman vortices resonance wouldn't occur with other components of the unit under positive conditions. The research results provided references for optimization design of blade tail shape under the reverse condition in bidirectional turbine.