We propose a metasurface which consists of three conductive layers separated by two dielectric layers.Each conductive layer consists of a square array of square loop apertures,however,a pair of comers of each square metal patch surrounded by the square loop apertures have been truncated,so it becomes an orthotropic structure with a pair of mutually perpendicular symmetric axes u and v.The simulated results show that the metasurface can be used as a wideband transmission-type polarization converter to realize linear-to-circular polarization conversion in the frequency range from 12.21 GHz to 18.39 GHz,which is corresponding to a 40.4％ fractional bandwidth.Moreover,its transmission coefficients at x-and y-polarized incidences are completely equal.We have analyzed the cause of the polarization conversion,and derived several formulas which can be used to calculate the magnitudes of cross-and co-polarization transmission coefficients at y-polarized incidence,together with the phase difference between them,based on the two independent transmission coefficients at u-and v-polarized incidences.Finally,one experiment was carried out,and the experiment and simulated results are in good agreement with each other.