Disclosed are a system and method for controlling a thickness of a hot-dip galvanized coating on a strip material with a continuously varying thickness. The system comprises a steel strip thickness gauge (8), a length measuring roller (11), a hot-dip galvanizing apparatus, an air knife (5), a cold-state zinc coating measuring instrument (7), and a control system (10). In a forward movement direction of a steel strip (1), the steel strip thickness gauge (8) and the length measuring roller (11) are arranged before an inlet of the hot-dip galvanizing apparatus, and the air knife (5) and the cold-state zinc coating measuring instrument (7) are sequentially arranged behind an outlet of the hot-dip galvanizing apparatus. The steel strip thickness gauge (8) detects a thickness signal of the steel strip (1), the length measuring roller (11) detects a length signal of the steel strip (1), the cold-state zinc coating measuring instrument (7) detects a zinc coating thickness of the steel strip (1), and the steel strip thickness gauge (8), the length measuring roller (11) and the cold-state zinc coating measuring instrument (7) respectively send measurement data to the control system (10). The control system (10) predicts a thickness of the steel strip at the air knife (5) and adjusts a pressure of the air knife (5) according to the length signal and the thickness signal, and further adjusts the pressure of the air knife according to the zinc coating thickness.