In this work, a detailed theoretical analysis shows that any polarized wave can be regarded as a composite wave composed of two orthogonal linear-polarized (LP) components with equal amplitude, and different polarized waves only have different phase differences Δϕuv between the two LP components; to modulate the polarization of an arbitrary polarized wave, it is only necessary to change the phase difference Δϕuv between the two LP components. According to the analysis conclusion, a polarization conversion metasurface (PCM) with an elliptical conductor patch as unit cell is proposed, it is an orthogonal anisotropic structure symmetrical to both U- and V-axes, and the phase difference Δφuv between the reflection coefficients ruu and rvv can be close to any constant in the frequency range from 7 to 23 GHz by changing the direction and size of the elliptical conductor patch in its unit cell. Thus, the PCM can be used to modulate the polarization of arbitrary polarized waves. The simulation and experimental results prove that the PCM can convert arbitrary polarized waves into linear polarized and circular polarized waves in the frequency band of 7.10–23.02 GHz by changing the direction and size of the elliptical conductor patch in its unit cell, moreover, it has a very good polarization conversion rate when the phase difference Δφuv is not very large.