Smoothed particle hydrodynamics (SPH) is a fully Lagrangian particle method which solves a problem without using any mesh or grid. Pressure fluctuation is one of the main drawbacks of the weakly compressible SPH (WCSPH) method that leads to an inaccurate pressure distribution. In the present work, a diffusive term is added to the continuity equation to suppress the density and consequently pressure fluctuations. In contrast to the mesh-based methods, flow separation and inflow/outflow boundary conditions are two challenging issues in the SPH method. To overcome these problems, a new algorithm for inflow/outflow boundary condition as well as a particle shifting method is utilized for simulation of flow past a cylinder. Comparing the results with those of literature, it is shown that the method is capable to decrease the pressure fluctuations and solve problems including open boundaries as well as flow separation.