〗Objectives: The reservoir heterogeneity is the key point of the reservoir. The low permeability and tight reservoir of Shan- 2 （the 2nd Member of the Shanxi Formation, Lower Permian) reservoir in the Ordos Basin has servous reservoir heterogeneity. the flow unit distribution of the reservoir is the most intuitive of the reservoir heterogeneity. The main aim of the paper is to show the flow unit of the Shan- 2 reservoir in the Ordos Basin and discuss its formation reasons. Methods:To take one hundred wells of A gas field Porosity and permeability data and reservoir parameter data into consideration, the FZIand seepage index are calculated. the flow unit distribution of the Shan- 2- 3 ( the 3rd sand bed, 2nd Member, Shanxi Fm. , Lower Permian) reservoir is given according to these result. Through general geological analysis, the formation origin of the flow unit is given. The discuss is tested by dynamic data. Results: It demonstrate that flow unit distributon is zonation along the channel sand body, and block vertical to the channel sand body. furthermore, the formation mechanism of the phenomenon is studied. It shows that the plane heterogeneity, which is result from the different channel body and the physical differences between the main channel and the edge of the channel, control the zonation of flow unit distributon, and the inerlayer heterogeneity, which is come from delta sand body model progradation and Meandering riverside stall development, control block ditribuition of the flow unit. Conclusions: The study area can be divided into fifteen flow units,in which, the main flow units are composed of 1 and 2 type flow unit. They have good reservoir permeability and connection, and they are in linear seepage, having high production. The second flow units are composed of 3 type flow unit. They are in nonlinear seepage with starting pressure gradient, having low gas production. Different channels in different places, different microfacies in the river channels, the overlay parts in the upstream and downstream channel, all these facts not only form the strong heterogeneity, but also form the zonation and block ditribuition of the flow unit.