Abstract: Geotextiles are widely used in people's living and project construction, mainly for water proof, filtration, heat preservation, isolation and many other aspects. The percent open area is one of the most important parameters which reflect the performance of geotextiles. The change laws of the percent open area in unsymmetrically biaxial tension tests are explored through the imagine analysis, and the results are more applicable than those of the uniaxial tension tests and symmetrically biaxial tension tests by the previous researchers. The difference between the theoretical and experimental values is compared, and the idea of “Projective Poissons ratio” is proposed.