The invention discloses a microwave signal photonic frequency conversion and multichannel fiber transmission device and method realized by a polarization multiplexing modulator. The device, as shown by a figure, comprises a laser diode (LD), a polarization multiplexing Mach-Zehnder modulator (PDM-MZM), an erbium doped fiber amplifier (EDFA), a 1:N light splitter (Splitter), a polarizer (Pol), a polarization controller (PC), and a standard single-mode fiber (SMF) and a photoelectric detector (PD). The PDM-MZM is used to carry out carrier suppressed double side band modulation on radio frequency (RF) and local oscillation (LO) signals and then implement frequency mixing, and the RF and LO isolation degree of frequency conversion gain and frequency conversion signals is improved. Optical signals output by the modulator can be divided into multiple paths to implement fiber transmission, each path is adjusted via an independent polarization state, and power attenuation caused by dispersion can be compensated.