The utility model discloses a continuous constant-pressure filter device for oil field water injection, which is designed for sufficiently utilizing pressure of filtered water, and avoiding arranging a clear water tank and a water feeding pump. After being in parallel connected with two booster pumps, a raw water pipeline of the filter device is divided into a header exceeding pipeline connected with a gas-pressure water tank and a water inlet header tube connected with each electric three-way valve and a filter; each electric three-way valve is communicated with a blow-off pipeline and a water outlet header tube; a filter-screen filter which is connected with the exceeding pipeline, the blow-off pipeline and a two-stage filter water outlet pipeline is arranged on the water outlet header tube; the two-stage filtration outlet water pipeline is communicated with a water injection pump pipeline and a gas pressure water tank. An intermediate water pond or a water tank does not need to be arranged for the filter device disclosed by the utility model, the filter is used for filtering while back flushing without cutoff, so that pressure of the filtered water is constant, and water yield and water injection amount of the filter can be kept matched all the time. Buffer energy storage of equipment can ensure that the filter device stably operates when a water injection pump is started and stopped and flow rate is regulated. The exceeding design can ensure that the filter device safely operates during maintenance and when an accident happens.