The invention discloses a spatial tower leg structure testing method of a narrow-based power transmission steel pipe tower. The spatial tower leg structure testing method particularly comprises the following steps: (1) keeping the slope of a main material to be constant, and changing the foot distance and the specification of the main material to change the included angle of main and diagonal materials of a tower leg and the slender proportion of the main material; (2) cutting out a tower leg part and designing and manufacturing a test model on the basis of a whole adjusted tower model; (3) determining the loading magnitudes of all test pieces according to the similarity criterion required by a test model design; and (4) amplifying or reducing loads of the original tower by certain times for researching the secondary stress intensity of the main material of the tower leg in a 100% design load state, that is, the main material of the power transmission tower leg is close to the full-using rate after determining the specification of the main material, and thus, the adverse effect of secondary stress generated by the joint rigidity is fully considered in the practical narrow-based power transmission steel pipe tower, the design margin can be accurately controlled and the design is relatively safe and relatively economic.