The invention discloses a layered in-situ loess collapsibility coefficient testing device and method. The layered in-situ loess collapsibility coefficient testing device comprises a support, a force measuring ring, a force transferring mechanism and a pressure bearing mechanism, wherein a jack is arranged on the support; the pressure bearing mechanism comprises a settling plate and a bearing plate which are positioned on the bottom surface; a permeable stone is mounted between the bearing plate and the settling plate; permeable holes are formed in the bearing plate and the settling plate. The layered in-situ loess collapsibility coefficient testing method comprises the following steps: digging an exploratory well, mounting the pressure bearing mechanism at one end of the force transferring mechanism, putting the pressure bearing mechanism into the exploratory well, mounting the force measuring ring at the upper end of the force transferring mechanism, loading the force measuring ring by the jack in a grading manner till the maximum load is reached, flooding and observing additional settlement, recording the additional collapsibility amount of the loess layer, increasing the depth of the exploration well, repeating the steps to obtain the additional collapsibility amount of each loess layer, calculating the loess collapsibility coefficient, calculating the loess collapsibility amount by referring to a national standard, and evaluating the loess collapsibility. By the layered in-situ loess collapsibility coefficient testing method, the disturbance of the original shape of soil in the sampling process is overcome, a test result is very close to an actually measured result of a flood test, and the test period is short, the expense is low and the water consumption is low, so that the layered in-situ loess collapsibility coefficient testing method has a very high popularization value and a very high application value.