A computer casing structure is composed of a chassis, which is provided with a front panel on which is mounted with a lower long through-hole and an upper long through-hole; a metallic shield, which is punched with a plurality of through-holes and is locked and positioned into the lower long through-hole; a CD-ROM module, which is positioned at a rear side of the upper long through-hole; and a main board, a front edge of which is provided with a plurality of I/O connection ports, and which is fixed in the chassis, with the I/O connection ports being corresponding to the through-holes of the metallic shield. By mounting the CD-ROM module, a hard disk drive module, and I/O connectors on the same front panel and installing a data transmission port on a rear panel, convenience in operation is developed, and an effect of ventilation and heat dissipation is improved.