The invention provides an ultra wide band low radar cross section microstrip antenna and aims to solve a technical problem of narrow bandwidth of a microstrip antenna reduction radar cross section inthe prior art. The microstrip antenna comprises a medium substrate, a metal floor, a polarized conversion surface, a radiation unit and a coaxial connector, wherein the metal floor is printed at a lower surface of the medium substrate, the polarized conversion surface is printed at an upper surface of the medium substrate, the center position of an upper side of the medium substrate is provided with a rectangular chamber, an upper portion of the chamber is provided with the radiation unit fixed with an output end of the coaxial connector, the polarized conversion surface is composed of four polarized conversion unit sets, each polarized conversion unit set comprises multiple decreasing fractal units, and arrangement direction difference of decreasing fractal units of the adjacent polarizedconversion unit sets is 900. The microstrip antenna is advantaged in that the ultra wide band decrement microstrip antenna radar cross section has stable performance, and the microstrip antenna can be applied to antenna systems with the complex electromagnetic environment to reduce electromagnetic interference among antennas in a wide band.