Abstract: A biporous system was analyzed, and the results show that specimens under the same matric suction have different equilibrium saturations because of the random distribution of their pore structures. The water-air distribution of an ideal unsaturated soil was analyzed, and the results show that the mechanical behaviour of unsaturated soil is affected by the difference in equilibrium saturations. The above results were further confirmed by pressure plate test and direct shear test. Results show that there are differences in equilibrium saturation, and that these differences decrease with increasing matric suction. Specimen shear strength decreases with increasing equilibrium saturation for the same matric suction. Further fitting results show that the internal friction angle related to matric suction is significantly affected by the equilibrium saturation: the maximum difference is about 45.18% of average value. Based on the test results and mean value, the following test structure is proposed: multiple parallel tests should be done under both the minimum and maximum matric suctions, and then the mean value obtained. This format can help to improve test results and save time.