Abstract: Based on the seepage theory,the formulas of interwell pressure and pressure gradient under different injection-production well patterns,namely injection-production pattern,the combination well pattern and the vertical well pattern,for the horizontal wells have been derived,and the pressure along various well patterns and the pressure gradient distribution regularity have been analyzed.The determination method for the critical injection- production well spacing of low permeability reservoir has been established and its influencing factors have also been analyzed.The results indicate the displacement pressure gradient of the vertical well pattern is bigger around the injection-production wells,while when the injection-production well spacing is bigger,the pressure gradient is relatively smaller.Its displacement pressure gradient is approximately 3~4 times as much as the vertical wells pattern.It is easier to create an effective displacement for combination pattern compared to vertical wells pattern,and it can take greater injection-production well spacing.