Abstract: It is a fascinating field for elucidating and quantifyingthe tactual performance of object softness in sensory engineering. In termof the macroscopically contacting interaction between fingerpad and objectsurfaces as well as the touching means, an equivalent biomechanical model isdeveloped to simulate the process of cutaneous low-thresholdmechanoreceptors sensing object softness. And then, two indexes whichcharacterize the ability of human tactual system in detecting anddiscriminating object softness, namely mechanical sensitivity and perceptualsensitivity are established on the basis of the developed biomechanicalmodel. By the established indexes, it is discussed parametrically that thegeneral mechanistic principle that human tactual system processes theproperty of object against compression. With the former discussion intheory, it is concluded that the mechanical resistance of object againstcompression is the tentative potential physical property on which humantactual system discriminate the softness of objects. Furthermore, thedecetability and discriminability of human tactual system in object softnessdepends on the ratio of the mechanical resistance against compression ofobjects to that of soft tissues within fingertip, and on the touching forceexerted by fingertip. Meanwhile, in terms of the deformable medium of softtissues within human fingertip communicating mechanical resistance of thedetected objects against compression, the working principle of human tactualsystem is different from that of the man-made testing machine with adesigned sensitivity, and human can not discriminate from all of compliantobjects with significant instrumental differences by compression testers.All of conclusions will help to understand the variation of estimation onfabric softness by touch means, and to improve the performance of predictionmodel and virtual rendering of object softness.