The invention discloses an existing tunnel lining reinforcement structure and a reinforcement method. The reinforcement structure is characterized in that arch lining steel frames serve as main supporting structures and are adopted for supporting formwork to form supporting units, a distance between the inner side of each supporting unit and an existing tunnel lining is smaller than 37.5cm to guarantee catenary safety and construction safety, concrete is poured to the inner side of the supporting formwork repeatedly, and the lining steel frames and the formwork are demounted after pouring is finished. According to the reinforcement method, the existing tunnel is equally segmented, segments are sequentially numbered, the supporting units are erected at intervals on odd segments, width of the supporting units is determined according to the segmenting conditions and stress conditions in a concrete pouring process, each supporting unit forms an independent concrete pouring cavity, concreteend faces of odd segments serve as end moulds of the even segments, and only inner side formwork supporting is needed; by segmented pouring, the lining steel frames can be demounted conveniently, influences on concrete service life due to pouring of the lining steel frames in concrete are avoided, and operating efficiency is greatly improved.