The polarization characteristics of Travelling-wave semiconductor optical amplifier (TW-SOA) is investigated based on th gain property of active Fabry-Perot resonator. Theoretical analysis and experimental research of polarization sensitivity is presented including the influences of residual facet reflectivities, gain coefficient, optical mode confinement factors, etc. Some improving methods for polarization sensitivity are discussed according to the analysis and a novel polarization insensitive TW-SOA is proposed.