Multiphase flow in a typical horizontal gravity separator on some offshore platform was simulated and RNG k-e model was adopted.Distributions of both oil phase and water phase at different time were obtained by numerical simulation.Results show that separation process will be affected by type and position of inlet.Measurable result of affected area was also obtained by numerical simulation.Simulation results coincide with theoretic analyze results.It is also proved that numerical simulation method could be applied to simulate multiphase flow in separator.