The invention relates to the field of auto parts, in particular to a sheet structure of an air inlet of a plastic air intake manifold for a four-cylinder gasoline engine. The sheet structure comprises an upper sheet inlet end and a lower sheet inlet end; the upper sheet inlet end and the lower sheet inlet end are mutually welded to form a cylindrical pipeline structure. The sheet structure is characterized by further comprising an upper sheet insert; the upper sheet insert is a circular arc-shaped opening component fixed on the circumference of the inner side of the upper sheet inlet end, and the opening end of the upper sheet insert is attached onto the circumference of the inner side of the lower sheet inlet end and surrounds with the inner wall of the lower sheet inlet end to form a smooth pressure-stabilizing cavity inlet pipeline. According to the sheet structure provided by the invention, the frequency of welding is reduced, the reliability and the service performances of products are ensured, the welding is extremely convenient, the production efficiency is improved, therefore, a great popularization value is realized.