The invention discloses a well group synchronous separated injection and production water injection well water injecting amount splitting method and device and belongs to the technical field of oil production. The method comprises the steps that the distance between a water injection well and the supply edge of a stratum is calculated through iterative computation with a first formula, a second formula, a third formula and a fourth formula, wherein the supply edge of the stratum is located between the water injection well and an oil production well communicated with the water injection well; the split coefficient of the oil production well is calculated according to a fifth formula and the distance between the water injection well and the supply edge of the stratum, wherein the split coefficient is used for indicating the proportion of water, injected to the injection well, flowing to the oil production well. According to the well group synchronous separated injection and production water injection well water injecting amount splitting method and device, the split coefficient is obtained through calculation with the first formula, the second formula, the third formula, the fourth formula and the fifth formula so that technical personnel can reasonably inject water to the water injection well according to the split coefficient, and decreasing of the yield of the oil production well or a blowout accident can be avoided.