The invention discloses a construction method for preventing deflection of wide steel box girders of a cable-stayed bridge, belonging to the technical field of bridge construction. During cantilever splicing construction adopted in the invention, deflection of the steel box girders is prevented by changing the construction technology and timely and effectively correcting the deflection of the steel box girders in the construction process. The construction method is characterized by comprising the steps of: firstly correcting deflection of steel side boxes when the steel side boxes are hoisted so as to correct the deflection continuously derived from a previous section; and connecting 50% of bolt holes adopted on the splicing mouth parts of the steel side box at one side with punching nails so as to ensure the splicing mouths of the steel side box at the side to have moving margins and further facilitate installation of cross girders, effectively improve the stress conditions of the steel side boxes and prevent deflection of the steel side boxes. The construction method has the beneficial effects of effectively preventing the deflection of the steel box girders during construction and ensuring the construction quality.