Abstract: The flood-response time in low-permeability reser- voirs predicted with the conventional formula of pressure transmitting coefficient deviates largely from its actual value. There exists the starting pressure gradient in low permeabili- ty reservoirs,and the flow boundary changes in the time- variant seepage.In view of the starting pressure gradient, the pressure distribution of the time-variant seepage is de- rived on the basis of the seepage flow formula with starting pressure gradient and the approximate equation of reservoir pressure.The relationship between the flood-response time and the well spacing is established in line with the principle of material balance.The pressure travel rime is closely corre- lated with the pressure gradient in low permeability reser- voirs,and there exists a positive correlation between the flood-response time and the starting pressure gradient.The formula of flood-response time presented in this paper pro- vides a significant method to predict the flood-response time and to arrange the well spacing.