A closure cover and an extension ladder are provided. The closure cover includes an upper closure plate, a lower closure plate, and two airtight sheets, the upper closure plate and the lower closure plate are fixedly connected by a connector, the upper closure plate and the lower closure plate are each provided with ventilation holes, the airtight sheets are fixed to a surface of the upper closure plate facing away from the lower closure plate and a surface of the lower closure plate facing away from the upper closure plate, respectively, and the airtight sheets are capable of controlling the closing and opening of the ventilation holes.