We demonstrated a method to trim the beat frequency of dual-polarization fiber grating lasers by side-exposing the laser cavity to UV beam. The UV-side-illumination induces an additional birefringence of the cavity fiber and therefore permanently changes the beat frequency of the laser. The beat frequency can be trimmed up or trimmed down, depending on the UV incident direction relative to the intrinsic polarization axis of the active fiber. A trimming range as much as ~700MHz has been achieved. This method allows us to actively control the beat frequency of dual-polarization fiber grating lasers. A 6-channel RF-frequency division multiplexed polarimetric fiber grating laser array has been demonstrated.