An octagonal- and variable-section steel pipe cross arm for steel pipe poles comprises a cross arm body, a line hanging point device and a connecting flange. The section of the cross arm body is regularly octagonal; the lengthwise section of the cross arm body is trapezoidal; the front end of the cross arm body is provided with the line hanging point device which is horizontal; the rear end of the cross arm body is fixedly connected with the connecting flange; a passage is arranged on the cross arm body. The octagonal- and variable-section steel pipe cross arm has the advantages that the cross arm body is made of an octagonal- and variable-section steel pipe, only one longitudinal joint along the length of the steel pipe needs to be welded, processing amount is greatly reduced, and the cross arm is stably and reliably connected with a pole through the flange.