A novel phase shifter for high power microwave is described and analyzed. The coaxial TEM mode is converted to TE11 fan-shaped waveguide mode by inserting fins into the coaxial waveguide.The phase shift changes with varying the length of the fins. A phase shifter for 4 GHz based on this principle is designed and optimized. The calculated results show that the maximum phase shift is 360° when the length of the phase shifter is 50 cm, the inner radius 2.0 cm and outer radius 4.5 cm, and the loss is below 0.1 dB in the range of 3.9~4.1 GHz. The input and output of phase shifter are co-aligned on the same axis.