Abstract: Responding time of waterflooding in low permeability reservoirs predicted with conventional pressure transmitting coefficient formula deviate largely from its actual value. In viewing that start-up pressure gradient exists in low perme ability sandstone reservoirs, a radial drainage production formula and pressure distribution formula during stable pro duction are derived on base of the seepage flow formula with start -up pressure gradient, and the relationship be tween responding time of waterflooding and well spacing are established by using material balancing method. Pres sure wave propagation time is closely correlated with pres sure gradient in low permeability reservoirs, and responding time of waterflooding is directly proportional to start -up pressure gradient and. cubic well spacing. The actual re sponding time of waterflooding is coincident with the cal culation results with this method in Baozhong block.