The invention belongs to the technical field of tunnel engineering and particularly relates to a supporting and retaining structure of an existing retaining wall straight penetrated by a large-span tunnel and a construction method thereof. The supporting and retaining structure comprises two rows of anti-skid piles, a ground coupling beam, coupling beams, a top beam, inclined struts, an anchor stay cable structure and a grouting reinforcement body. The two rows of anti-skid piles are mounted on the outer side of the supporting and retaining structure. The front row of anti-skid piles are at a certain distance from the supporting and retaining structure and are connected with one another through the ground coupling beam. The rear row of anti-skid piles are tightly attached to the supporting and retaining structure and connected with one another through the top beam, the coupling beams and the ground coupling beam. The two rows of the anti-skid piles are connected through the ground coupling beam and the inclined struts to form an overall frame structure. The rear row of anti-skid piles are provided with opposite-pull anchor stay cables. Rock and soil masses on the stressed sides of the peripheries of the anti-skid piles are subjected to grouting reinforcement, and the bearing capacities of the rock and soil masses and the supporting and retaining structure are improved. The supporting and retaining structure is applicable to an entrance and an exit of the large-span tunnel and a tunnel entrance heading slop with the requirement of small deformation, the reinforcing effect is guaranteed, and traffic operation is not blocked.