By means of Eulerian Model combined with Discrete Phase Model, the numerical simulation of three phases flow field in bubble enhanced hydrocyclone was performed to analyse the velocity distribution inside the hydrocyclone and the trajectories of oil droplet. The separation efficiency of hydrocyclone after bubble injection was compared with conventional hydrocyclone in different aspects such as partical size, flow rate and oil content in wastewater. The simulation result showed that tangential velocity and axial velocity inside the hydrocyclone after bubble injection were higher than that of conventional hydrocyclone, leading to a quicker escaping time of oil droplet. Meanwhile, bubble injection could increase the separation efficiency of oil droplet above 30 micron by 10% to 20%. The calculation confirmed that bubble injection at inlet could promote the separation performance of hydrocyclone in terms of numerical simulation.