According to the seismic design code for buildings in China, the soil formed during or before the late Pleistocene will not be liquefied. However, case histories and laboratory test results show that the saturated late Pleistocene loess is potentially vulnerable to liquefaction. Using a cyclic triaxial apparatus with a bender element system, the shear wave velocities of undisturbed loess are tested continuously during sample saturation. The test results indicate that the shear wave velocity of loess in the saturation process is rapidly reduced in the first 10~20 min, and then, it becomes stable. Shear wave velocity is a parameter that accounts for the small strain shear stiffness of soil, which accurately characterizes soil structure. The rapid reduction of shear wave velocity indicates that the microstructure of the loess is destroyed by the sample saturation process. The stable shear wave velocity indicates that the saturated loess has a new stable microstructure. The above analysis proves that the saturated loess is different from the loess before saturation. Indeed, only loess that has been subjected to agricultural irrigation or groundwater rise is liquefied. Because the structure of the saturated loess differs from that of the original loess, different seismic design regulations should be applied as well. Therefore, the seismic design code for buildings in China should be adapted to consider the level of saturation of the loess in addition to its age. Furthermore, the saturated loess is no longer the late Pleistocene loess, and the geological age of liquefied loess may be consistent with seismic design code of buildings in China. We observed that clay particles exist in some loess as clay lumps, and clay lumps do not affect the liquefaction of other silty parts of the soil. However, in the clay content test, if the soil sample contains clay lumps, the clay content test result will be comparatively high and inconsistent with the static and dynamic mechanical behavior of the soil. Consequently, clay lumps should be removed from the clay content test sample, and then, the remaining clay content should be used to evaluate the soil's liquefaction potential.