The invention discloses a well mouth test blowout preventer for a water injection well. The well mouth test blowout preventer mainly comprises a blowout-preventing pipe; a blowout-preventing box is arranged at the upper end of the blowout-preventing pipe; the lower end of the blowout-preventing pipe is connected with a sealing pin; the blowout-preventing pipe and the sealing pin are connected into a whole, then are arranged on a lifting device in a sleeved mode and are arranged on or dismounted from a well mouth through the lifting device. A driving device for lowering and lifting a test instrument is arranged on the blowout-preventing box in a sleeved mode. The blowout preventer can effectively improve the test success rate of a high-pressure water injection well, and meanwhile the test accuracy and the water distribution percent of pass can be improved. The problem of testing of the high-pressure water injection well is solved, and the blowout preventer has the higher application and popularization value in testing of a normal-pressure water injection well. In a word, the well mouth test blowout preventer has the advantages that ground hydraulic lifting is adopted by the blowout preventing pipe, and worker climbing or crane working is removed; gap flow leakage and pressure reduction and secondary self-sealing blowout preventing are adopted, and well mouth miss is reduced; a motor drives a steel wire to overcome underground high-pressure strong descending and automatic continuous conveying.