Based on Ningbo rail transit Line 4 geotechnical investigation project, comparative lab-testing was conducted on soft soil taken at two different diameters (D= 75mm, D= 100mm)with the open thin wall samplers commonly used in soft soil areas. Based the test results, the differences of indexes of physical and mechanical properties of soft soil at two sampling diameters and their mechanism were analyzed; the additional volumetric strain method was used to analyze and evaluate the disturbance of soil samples at two different diameters. The results show that the water content and void ratio of the soil sample with the diameter of 75mm reduced to a certain extent with the reduction range of 9% compared with the soil sampler with the diameter of 100mm; the density, compressive modulus and shear strength index increased to a certain extent with the increasing of 2% to 21%; the average volumetric strain of soil samples was 6.4% and 6.6% respectively, and the degree of disturbance was very large; the soil samples which were obtained by the large diameter soil sampler was closer to the actual properties of soft soil than the small diameter soil sampler.