Abstract: The start-up pressure gradient can be obtained from the experiment of single-phase fluid in the low permeability reservoir.On the basis of the nonlinear flow function of start -up pressure gradient,the power function between the start -up pressure gradient and the mobility in the low permeability reservoir are deduced from the statistics and fitting of experimental data.By combining the filtration theory with the field application in the low-permeability oilfield,acquired are the expression of the limit projector-producer distance in low permeability reservoirs and the theoretical plates between the limit projector-producer distance and the permeability in different production pressure differential, which offer the theoretical basis to the well distance density.