The starting pressure gradient and pressure sensitivity effect proposed by Bear（1972） and Fatt（1952） respectively have practical significance for the development of low permeability oil and gas reservoirs. The so-called starting pressure gradient refers to the critical pressure gradient that causes the oil and gas formations to start flowing. The so-called pressure-sensitive effect includes the phenomenon that the formation permeability decreases as the formation pressure drops. Due to the theoretical needs of tight and low-permeability reservoir development analysis in the Ordos Basin, relevant experts and scholars in China have paid attention to the starting pressure gradient and pressure sensitivity effect for nearly 20 years, but their understanding is still in the initial perceptual stage. The reason is the lack of basic derivation. The derivation in this paper shows that the pressure gradient and starting pressure gradient of Darcy linear flow are constant, but the pressure gradient and starting pressure gradient of Darcy plane radial flow both are functions of radial radius. It is incorrect to directly apply the constant starting pressure gradient of linear flow to the plane radial flow equation. Although the pressure sensitivity effect of permeability exists, it can never be used in the flow equation of Darcy’s law. Because constant permeability is the basis for the establishment of Darcy’s law. Otherwise, it will shake the theoretical foundations of Ground Fluid Dynamics, Petroleum Reservoir Engineering and Reservoir Numerical Simulation.