The invention provides a method for avoiding crack formation of medium-high carbon steel during hybrid welding. The method comprises the steps that a vacuum chamber and a casting blank are preheated till the temperature of the casting blank reaches 290-310 DEG C; the welding speed of a hybrid welding gun is set to be 7.9-8.2 m/s, and the welding current is set to be 340-360 ma; a short edge is welded on the portion below a pre-welded weld joint at the end of the casting blank in parallel before welding; after the two ends and one edge part are welded, the other edge part is welded to be closed after 1.9-2.1 hours, and a long edge is welded to the lower portion of the other edge part in parallel; and a composite plate blank continues to be placed in a closed welding machine room for 12-15 hours after welding is finished. By the adoption of the method, longitudinal cracks generated in the vacuum hybrid welding process of the medium-high carbon steel are effectively avoided, so that the treatment workload is reduced, the production cost is reduced, and the quality of a composite steel plate is greatly improved.