Abstract: A coaxial Blumlein line with a spiral inner and a spiral outer cylinders is proposed in this paper. The pulse forming process for this line is analyzed via the transmission theory of transverse electromagnetic wave. It is found that the electromagnetic wave can transmit independently in the inner line and in the outer line which is the same as that for the conventional Blumlein line. The analytical expression for the parameters such as the wave impedance and the slow-wave coefficient are presented. The proof-of-principle experiments are also conducted, which show that the top of the output pulse is flat and is about 80% of the full width at half maximum (FWHM). The experiments also show that the amplitude and the FWHM of the output pulse are in line with the theoretical values, which verifies the feasibility to produce long pulse with this type of line. The weakness of this type of line is that it tends to be affected by the peripheral environmental conductors. When the distance between the peripheral environmental conductors and the Blumlein line gets longer, this unfavorable effect is weaker. By placing a ferrite core between the bottom of the spiral outer cylinder and the ground flat conductor, the distance between them can be decreased from 500 mm to 100 mm.