The invention discloses a polycarboxylic acid water reducer with a modified sugar side chain structure. The general structural formula of the polycarboxylic acid water reducer is shown in the description, wherein a, b, c, d, m and n are polymerization degrees, and are positive integers; a is equal to 8 to 0; b is equal to 1 to 12; c is equal to 50 to 140; d is equal to 1 to 20; m is equal to 45 to 76; n is equal to 9 to 27. The invention further discloses a preparation method of the polycarboxylic acid water reducer. The synthesized polycarboxylic acid water reducer molecularly has an amidated sugar side chain structure, which is more stable in the alkaline environment of concrete compared with an esterified sugar side chain structure, and does not undergo any hydrolysis reaction; after the polycarboxylic acid water reducer is added into concrete, high thickening stability is achieved, excellent water retention and slump resistance are achieved, and the fluidity of net slurry changes slightly after 120 minutes.