The invention provides a method for improving cracking resistance of a large-span continuous rigid frame bridge and a box girder of a combined structure. The method includes the following steps of 1,creating a finite element model of the rigid frame bridge, conducting analogue analysis and predicting the positions of cracking segments of a box girder web; 2, adopting the positions of the crackingsegments as the positions where steel-concrete combined webs are arranged, designing sections of different segments of the bridge again, and conducting analogue simulation on the stress state of thebridge again to determine the size of the box girder; 3, conducting checking calculation to determine whether or not the shearing resistance and rigidity of the box girder and the midspan deflection of the bridge meet design requirements, modifying the size if not, repeating step 2 till the requirements are all met and then determining the construction size; 4, according to the construction size,arranging toothed steel plates on the outer surfaces of web rebars at the predicted positions, connecting rebar teeth on the toothed steel plates with the web rebars, pouring concrete to enable the web rebars and the toothed steel plates to be solidified into the steel-concrete combined webs, and finally forming the steel-concrete combined box girder.