The invention relates to the field of mechanical designing and manufacturing and discloses an air inlet channel structure of a hydraulic retarder shell. An internally-arranged arc structure is adopted to be connected with the hydraulic retarder shell to be a whole. The problem that air inlet channels of existing retarders inside and outside the country are bended, and accordingly occupied space is large is solved. The arc air exhausting structure can not only meet retarding air inlet, but also can increase the strength of a working cavity through reinforcing ribs. The other characteristic of the air inlet structure is that a local thin wall is adopted, so that radiating and weight reducing of the retarder are facilitated; and a local thick wall is adopted, so that the strength and using performance of the working cavity of the retarder are increased, and accordingly the double functions of the local thin wall and the local thick wall are manifested.