The invention belongs to the technical field of radar stealth, and particularly relates to a wideband spirally-coding radar cross-section (RCS) reducing metasurface insensitive to polarization and a design method thereof. The metasurface is of a two-dimensional limited size structure and is formed by arranging eight types of linear super-elements having different gradient directions in 8 by 8 according to a spiral sequence, the gradient directions of eight types of linear super-elements are respectively 0 degree, 45 degrees, 90 degrees, 135 degrees, 180 degrees, 225 degrees, 270 degrees and 315 degrees, each linear super-element is of a two-dimensional limited size structure and is formed by six types of artificial electromagnetic structure units having different sizes in 6 by 6 according to linear gradient, the six types of artificial electromagnetic structure units realize 360-degree coverage, the phase gradient is 60 degrees, the spiral sequence is a single spiral circulation arrangement mode from outside to inside, each artificial electromagnetic structure unit is of a rotary symmetric reflection structure and comprises an upper-layer internal closed ring, an external Jerusalem metal structure, an intermediate-layer dielectric plate and a bottom-layer metal copper plate. The RCS reducing metasurface has the excellent characteristics of thin thickness, good robustness, ultra-wideband working and the like and is easy to process.