Abstract: Threshold pressure gradient exists in the development of low permeability reservoir. The process of CO₂ miscible flooding is very complicated, demanding further research. It can be divided into 4 zones including original oil phase zone, frontal phase zone of gas injection, miscible oil phase zone and trailing phase zone of gas injection. Indoor short core experiments are used to study the threshold pressure gradient of different flooding. Studies show that the pressure gradient-flow diagrams of low permeability core are in concave distribution which is divided to nonlinear flow and linear flow. The main factors of threshold pressure gradient are fluid viscosity, effective permeability, fluid density, gas oil ratio, and so on. The threshold pressure gradient and core permeability are based on power function, and the index is about -0.482.