An automatic control device for butt joint length of a space-frame pole is mainly formed in a manner that a positioning mechanism digital control system is electrically connected with a positioning mechanism through a cable. According to a length dimension of the space-frame pole, a pulse is transmitted by a controller of the positioning mechanism digital control system to a stepping driver, a stepping motor of the positioning mechanism is driven by the stepping driver to rotate, and a movable positioning component of the positioning mechanism is driven to shift along a guide rail, so that the length of the space-frame pole is accurately positioned, and automatic control on the butt joint length of the space-frame pole is realized; a length error of the space-frame pole is controlled to be within +/-0.3 mm; the length error value is reduced by 3 times in comparison with an error value specified by the national standard, and an overall accumulative error of a space frame can also be reduced by 3 times, thereby being very conductive to the space frame which is larger in length and complicated in shape. As the automatic control on the butt joint length of the space-frame pole is realized, auxiliary time can be shortened by 30 percent in operation, and the production efficiency can be effectively improved; moreover, the automatic control device for the butt joint length of the space-frame pole is simple in structure and convenient to operate.