A cooling device applied to a server or other casing reduces airflow resistance. A covered chassis defines a accommodating groove and includes two opposite ends, an air inlet located at one end and an air outlet located at the other end, the air outlet communicating with the air inlet. The cover of the chassis is mounted adjacent a side of the chassis and seals the accommodating groove. The cover defines first and second openings corresponding to the accommodating groove. Part of cooling air exits the casing through the accommodating groove and the air outlet, the rest of the cooling air exits the casing through the first opening, re-enters the casing through the second opening and exits the casing through the air outlet.