Polymer membrane with quality control on the silicon surface was synthesised, where the hard silicon was used as supportive substrate. The periodically metallic structures were patterned on the polymer film surface by normal photolithography. Finally, polymer film is peeled off from supportive silicon substrate. A large area and high uniformity of micro structures thin-film terahertz device has been obtained. A terahertz polarizer on polymide film substrate was fabricated using this method. Experimental results show that the polarization extinction ratio is more than 50dB at 0.1THz. The average transmittance reaches 89.78% in the polarization direction at the range of 0.1-2THz. The degree of polarization is over 99.84%. The polarization uniformity of the polarizer is fine around the diameter region of 42.3 mm. So, the flexible terahertz polarizer fabricated here is satisfied with practical demand.