The invention discloses a protecting device for a wellhead of a water injection well. The protecting device comprises a supporting foot fixing support, a hydraulic motor and a hydraulic oil cylinder,wherein the hydraulic motor and the hydraulic oil cylinder are fixedly installed on the supporting foot fixing support. A lifting ring is installed on the upper portion of the hydraulic motor; the bottom end of the hydraulic motor is connected with the top end of the hydraulic oil cylinder, and the bottom end of the hydraulic oil cylinder is connected with a sliding pipe through a wellhead connecting flange; the lower end of the sliding pipe is sleeved with a packer, a spring torque sleeve and a gauge cutter sequentially; and the bottom of the gauge cutter is connected with an anti-falling nutthrough a shearing pin. The protecting device is installed on the upper portion of the wellhead of the water injection well; the sliding pipe can be put downwards to the designated position below thewellhead, and setting of the packer is conducted so as to guarantee that liquid can be directly injected into the well without the need of passing through the wellhead; meanwhile, the wellhead protecting device can replace the wellhead to bear pressure, the wellhead pressure-bearing grade is improved, and the operation wellhead does not need to be replaced in the water injection well measure modifying process; and safe injection in water injection well measure operation is achieved, and the operation efficiency is improved.