Abstract: The producer?injector spacing corresponding to the minimum displacement pressure gradient overcoming the start?up pressure gradient in low permeability reservoir is the limit well spacing for establishing effective displacement pressure system between producer and injector. According to producer?injector model of aniso?production one?source and one?convergence point, a new formula for displacement pressure gradient in main stream line between producer?injector is deduced and applied to Fuyang reservoir in Xinmin oilfield which belongs to a typical low porosity and low permeability reservoir. This paper takes account of the real status of the reservoir development and determines the limit well spacing for it by this new method. Comparing common calculation methods available, the result shows it can be well applied in practice