The utility model relates to a detecting car for a main arch ring of a railway deck type arch bridge. The detecting car for the main arch ring of the railway deck type arch bridge is used for achieving the detecting to working condition of a driving condition main arch ring, checking and maintaining of the main arch ring daily, improving efficiency of checking and repairing greatly, and reducing labor intensity of maintainers. The detecting car for the main arch ring of the railway deck type arch bridge comprises a track (10), a walking trolley (20), an outer vertical frame (30) and a working platform (40), wherein the track (10) is fixedly installed on upper edges of two sides of the arch bridge main arch ring, the walking trolley (20) is arranged on the track (10), the outer vertical frame (30) is arranged on the two sides of the arch bridge main arch ring, the upper end of the outer vertical frame (30) is rigidly connected with the walking trolley (20), an actuating device (32) is arranged on the outer vertical frame (30), the actuating device (32) is connected with the walking trolley (20) in a driving mode through a driving mechanism (33), the working platform (40) is placed below the arch bridge main arch ring, and two ends of the working platform (40) are rigidly connected with the outer vertical frame (30).