The invention provides an extension building method of a small folding type 5G signal support tower. The extension building method comprises the following steps that firstly, an inspector starts a double-head motor; a first output shaft of the double-head motor drives a first screw rod to rotate around the axis of the first screw rod; the first screw rod drives an inner cylinder body to slide outwards along an outer cylinder body through a bottom cover plate until the built-in step is in contact with the external step; a support mechanism is switched to the extended state; in the first step, amounting mechanism extends outwards from the periphery of the inner cylinder body and is switched from the folded state to the unfolded state; in the first step, a second output shaft of the double-head motor drives a driving base to extend outwards from the opening position of the bottom end of the outer cylinder body, is switched from the hidden state to the expanded state and carries out landing vertical support on the support mechanism; and finally, the inspector manually fixes a 5G signal box on the mounting mechanism, so that the normal checking of the 5G signal by the inspector is met.The method has the beneficial effects that the size of the 5G signal support tower can be greatly reduced, and carrying and transportation of the inspector are facilitated.