An external circulation ball screw consisting of a screw shaft, a screw nut, a cover, two reflow members and multiple rolling members is disclosed. The screw nut defines a mounting groove on the outer perimeter and two return holes in the mounting groove. A first half-return channel on a first step in the mounting groove of the screw nut and a second half-return channel in a second step inside the cover constitute a return channel. Alternatively, a third step can be formed in the cover, or the screw nut can be configured to provide a mounting groove and a third step with return holes in the mounting groove. Thus, same size of reflow members can be used with different lengths of return channels.