Researching the movement of oncomelania in water flow is of vital significance in controlling oncomelania diffusion. In this paper, the open source software OpenFOAM for computational fluid dynamics is adopted to calculate the water flow past an oncomelania in two-dimensional viscous incompressible unbounded domain.Triangular unstructured meshes are generated to simulate two streamwise flows past an oncomelania. The finite volume method and PISO (Pressure-Implicit with Splitting of Operators) algorithm are used for solving the time-dependent two-dimensional incompressible Navier-Stokes equations and the wake flow is simulated at low Reynolds number. The detailed information of flow quantities including coefficients of drag and lift, vortex shedding Strouhal number, and wake formation length varying with Reynolds number is obtained. Research results suggest that the drag coefficient of oncomelania declines gradually with the increase of Reynolds number; wake flow displays three different forms. The scopes of the critical Reynolds number which marked the transformation of flow behavior from steadiness to instability are preliminarily determined.The research could serve as a theoretical basis for improving oncomelania control.