A mechanical disc brake structure contains: a base, two disc brake units, and a connection block. The base includes at least one elongated fixer, a first receiving space, two first connecting portions, two second receiving spaces, two limiting grooves two connection racks, two through orifices. Each disc brake unit includes a rotation portion, multiple first balls, a first circular disk, a second circular disk, an affix element, and a first cap. The connection block includes a first aperture configured to correspond to the second hole of the rotation portion of each disc brake unit so that each of two locking elements is connected with the second hole and the passing aperture; the connection block further includes a second aperture formed on a predetermined position thereof so that the steel cable is inserted into the connector from the second aperture of the connection block.