Equilibrium displacement is the basic principle for well pattern design in waterflood oilfield. For the case of buried hill reservoir with dominant mid-high angle fractures and large vertical thickness，the gravity of fluid has significant influence on equilibrium displacement in stereo well pattern. Based on percolation theory，taking five-spot stereo horizontal well pattern for instance，formulas for water breakthrough time of production well considering the gravity and that for injection-production well spacing while equilibrium displacement were established. The place of central horizontal well was calculated according to the selected oil reservoir parameters and was optimized using numerical simulation method. The results indicate that the error between calculation and numerical simulation is 4.73% which is allowable in experiment，and the reliability of the formula is verified. The water breakthrough time difference between vertical production wells decreases and then increases as the central injection well rises layer by layer. When the injection well of S2 locates in the 6th layer，equilibrium displacement is realized that the breakthrough time difference between S1 and S3 is the lowest. The degree of reserve recovery reaches 42.52%，which is the peak value，and is improved by 3.84% compared with that when S2 locates in 11th layer.When the equilibrium displacement is realized，the ratio of injection-production distance from S2 to S1 and that from S2 to S3 is 2.95. Therefore，well spacing adjustment could improve development by impairing non-equilibrium displacement induced by gravitational effect.