A pulley device for smooth sliding of sliding doors or sliding windows, comprising: a pulley component comprising a pulley; a connecting component comprising a rocker, one end of the rocker being rotatably connected to the pulley component; and a housing comprising a bottom plate and two side plates, the two side plates being fixed to two opposed sides of the bottom plate, respectively, so that the bottom plate and the side plates jointly define a receiving cavity configured to receive the pulley component and the connecting component, wherein a surface of the side plate facing the receiving cavity defines a receiving groove along a length direction of the side plate, and the other end of the rocker is received in the receiving groove and is capable of sliding along the receiving groove; wherein the side plate further defines a guiding groove, an extending direction of the guiding groove is not parallel to an extending direction of the receiving groove; the connecting component further comprises a positioning pin, the rocker defines a positioning hole; the positioning pin extends through the positioning hole; one end of the positioning pin is received in the guiding groove so that the positioning pin is capable of sliding along the guiding groove and guiding the rocker; wherein from an end adjacent to the bottom plate to an end far away from the bottom plate, the two side plates each comprises a first plate body and a second plate body; the receiving groove is located on the first plate body, and the guiding groove is located on the second plate body; wherein a distance between two opposed first plate bodies is less than a distance between two opposed second plate bodies. a pulley component comprising a pulley; a connecting component comprising a rocker, one end of the rocker being rotatably connected to the pulley component; and a housing comprising a bottom plate and two side plates, the two side plates being fixed to two opposed sides of the bottom plate, respectively, so that the bottom plate and the side plates jointly define a receiving cavity configured to receive the pulley component and the connecting component, wherein a surface of the side plate facing the receiving cavity defines a receiving groove along a length direction of the side plate, and the other end of the rocker is received in the receiving groove and is capable of sliding along the receiving groove; wherein the side plate further defines a guiding groove, an extending direction of the guiding groove is not parallel to an extending direction of the receiving groove; the connecting component further comprises a positioning pin, the rocker defines a positioning hole; the positioning pin extends through the positioning hole; one end of the positioning pin is received in the guiding groove so that the positioning pin is capable of sliding along the guiding groove and guiding the rocker; wherein from an end adjacent to the bottom plate to an end far away from the bottom plate, the two side plates each comprises a first plate body and a second plate body; the receiving groove is located on the first plate body, and the guiding groove is located on the second plate body; wherein a distance between two opposed first plate bodies is less than a distance between two opposed second plate bodies.