A computer system includes a chassis and an airflow blocking plate. The chassis comprising a first chassis sidewall, a second chassis sidewall, and a drive bracket which is used to mount a disk drive. The drive bracket is secured in the chassis and abuts the first chassis sidewall, and the first chassis sidewall defines a first ventilation hole. The air flows into the chassis via the first ventilation hole. The airflow blocking plate secured in the chassis and located between the first chassis sidewall and the second chassis sidewall. The airflow blocking plate prevents the air from flowing back to the drive bracket.