In this article, an ultra-wideband low radar cross section (RCS) antenna based on phase gradient metasurface (PGM) is proposed using the theory of characteristic mode. Firstly, an ultra-wideband unit structure is designed and arranged as a PGM, achieving an ultra-wideband RCS reduction. Secondly, the PGM are analyzed using characteristic mode theory subsequently, which provides the way to determine the position of the feeding port. Finally, the mode significance and mode weighting coefficient of this PGM-based antenna are further discussed for modifying the antenna design so as to enhance the in-band RCS reduction without affecting its radiation performance. The simulation results demonstrate that the proposed PGM-based antenna operates at the band from 8.96 GHz to 10.5 GHz, with a peak gain of 9.6dBi. Moreover, a RCS reduction is achieved from 8.08 GHz to 33.36 GHz, with a fractional bandwidth of 122%. A prototype of proposed antenna is fabricated and measured. The measurement and simulation results are in good agreement. Our work not only propose a PGM-based antenna with ultra-wideband low RCS, but also provides an efficient method for designing the high-gain slot antenna.