The invention discloses a carboxymethyl inulin graft polymer scale and corrosion inhibitor and a preparation method thereof. The main chain of the carboxymethyl inulin graft polymer scale and corrosion inhibitor consists of the following repeating unit (I): in the repeating unit (I), x:y=(20˜80):(20˜80); the range of z is from 2 to 70, the degree of carboxymethyl substitution is from 1.5 to 2.5. The beneficial effect of the technical scheme proposed in the present invention is: it has excellent scale inhibition and corrosion inhibition properties; the monomers are cheap and easily available; the reaction conditions are mild and easy to control; at the same time, the monomer raw materials don't contain phosphorus and nitrogen, and are environmentally friendly.