The invention provides a complex plug removal water injection agent used for a water injection well of a sandstone reservoir and a preparation method thereof, and discloses a self-synthesized organic cation low molecular weight polymer with well prevention of clay swelling and transport of clay particle, and excellent anti-erosion property. According to the invention, selected and used hydrofluoric acid, formic acid and acetic acid have well dissolution and corrosion properties for sandstone and retarding property, so that the action between the acid and the sandstone is slowed down, the depth of the entering of the acid is increased to be able to acidize deeply so as to effectively relieve the plug of rock particles and scales

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