The invention belongs to the technical field of oil region development drilling, and specifically relates to a continuous tube old well sidetracking method. The method comprises the following steps of(1) drift-diameter scraping a sleeve pipe of an old well until impurities on the wall of the sleeve pipe are cleaned up; (2) opening a window in the sleeve pipe after being cleaned in step (1); during opening the window, firstly selecting a window opening position according to a selection principle of the window opening position; then downwards feeding a window opening tool to the window openingposition, and milling for opening the window; and (3) downwards feeding a drilling tool assembly to drill the old well on which window opening is performed in step (2). The problems of difficulty in carrying sand, large friction resistance, severe blockage, low drilling efficiency and poor safety during continuous tube old well sidetracking are solved; and the method has the advantages of easinessin carrying sand during drilling, small friction resistance and small blockage during drilling, and the drilling efficiency and the safety are improved.