The invention provides a testing device and method for a true cohesive force of clay. The testing device comprises a water tank, wherein a clay sample to be tested is arranged in the water tank; a porous plate is placed on the upper end surface of the clay sample; and vertical displacements of the porous plate are measured through a dial indicator. The measuring method comprises the following steps: preparing the sample according to regulations for highway geotechnical tests; carrying out placement and submerging saturation on the sample; applying overlaying loads until the sample is damaged,obtaining the maximum main stress sigma<1> on the sample through measurement, and measuring an inclination angle theta of a shearing surface when the sample is damaged; and determining coordinates ofa Mohr's circle according to the Mohr-Coulomb theory, drawing the Mohr's circle, and drawing a tangent line B passing through an intersection point a of a straight line, which passes through an original point and of which the inclination angle is theta, and the Mohr's circle, wherein an intercept of the tangent line B on the Y axis is the true cohesive force of the clay sample. The device and method provided by the invention has the advantages of high simplicity, high convenience in operation and high reasonability in structure; and the true cohesive force of the clay in a status without a confining pressure can be obtained, so that the long-existing problem in the field that the true cohesive force of the clay cannot be accurately obtained for slope stability calculation is solved.