Based on the filtration theory, using the steady successive substitution method to divide the entire flow process into differentiation elements, assuming steady flow in each differentiation element, then substituting the stable production rate equation which is suit for the low-permeability reservoirs into each differentiation element, after integral calculation, combining with material balance equation could obtain the start-up pressure gradient. By using production data including production and pressure, this method break away from the traditional laboratory method to calculate the starting pressure gradient. At the same time this new method was applied to an oil field and compared with the measured starting pressure in laboratory, the results confirmed the reasonableness of the new method.