Abstract: Compared with conical and spherical indentation, flat indentation has a stable contact area and an obvious linear elastic stage, which are beneficial to realize the integrated characterization of material elastic-plastic parameters more stably. However, the current flat indentation testing methods of metal materials still have some shortcomings, such as large contact stiffness and high deformation loss of indenter. Therefore, it is necessary to develop a mechanical property testing method with smaller contact stiffness and higher prediction accuracy. A truncated cone flat indenter with a cone angle of 60° was thusly used, and the finite element analysis proved that it has a weak sensitivity to tip rounding compared with cylindrical flat indenter. Based on the equivalent energy principle, a theoretical model of truncated cone flat indentation (TCFI) was established, which is associated with the diameter of the indenter, indentation work, depth, and the elastic-plastic parameters of materials. A testing method for mechanical properties of materials based on the model was proposed, and the accuracy of the method was verified by the finite element calculation in a wide range of parameters and by the indentation test and tensile test of 5 metallic materials.