A door operator includes an installation box, an operating mechanism, a pneumatic apparatus and a moveable sleeve. The installation box has a housing and a lid. The housing has an assembling space and an opening communicating with the assembling space; the lid is mounted in the opening and covers the assembling space. The operating mechanism is rotatably mounted in the installation box and has a gear. The pneumatic apparatus is installed in the assembling space and has a cylinder and a piston rod. The moveable sleeve is movably mounted on the cylinder and connected with the piston rod and engaged with the gear of the operating mechanism. Because a rotation axis of the gear and a central axis of the moveable sleeve are perpendicular to each other, the volume of the installation box can be reduced.