Well pattern infill can improve the effect of oilfield development and enhance oil recovery. The denser the well spacing density is, the better the effect of oilfield development will be. There are manymethods for calculatingwell spacing density at present, but only a few methods are suitable for calculating the reasonable well numbers of well pattern infill and economic limit well number in water drive field. Based on В. Н. шелкачев formula, the net present value formula is applied to deduced the theoretical calculation formula for reasonable well numbers ofwell pattern infill and economic limit well number in water drive field. The application result shows that the calculation result is reasonable, and this method can provide theoretical support for well pattern infill in oilfield.