A series of tensile test are carried out for a warp-knitted composite geotextile with high tensile strength composed with polyester filament and polypropylene non-woven geotextile. The effects of width and the initial length on the tensile properties of the geotextile specimen is studied, and furthermore, two different computing methods of tensile strength, wide-width strip method of geotextiles and multi-rib tensile method of geogrids, are compared. Test results show that the wide-width strip method is more suitable than multi-rib tensile method for the calculation of composite geotextile. The tensile strength of composite geotextile is higher with the increasing of specimen width. When the specimen’s width is greater than 100 mm, the strength tends to be stable proximately. In addition, the effect of the initial length of geotextile specimen is not obvious; with the increase of length, the maximum tension force is only slightly lower, but the elongation rate reduces greatly, and the tensile strength corresponding to an elongation rate of 5% is improved greatly. In the tensile test of such geotextiles, the width should be 200 mm, and the initial length should not be less than 300 mm at least.