Under the action of tensile load obvious necking-down phenomena will accur in the metal test-piece. The tensile experiment about the metal test-piece has been studied, but the finite element simulation of the tensile necking-down phenomena has not been reported. The paper first studies the experiment of the mental test-piece, using the universal testing machine of WE-10A and WI-60T to load the mental test-piece and using IZ-910 infrared thermal imager to test the change process of infrared radiation temperature fields, then works out the finite element program to simulate the tensile necking-down phenomena of the metal test-piece with the elastic-plastic model. The experiment result shows that the infrared thermal effect of the metal test-piece in the process of loading is obvious, before the sample fracture will appear the omen of high temperature in the position where the future fracture will occur; in stress concentration the microcracks concentrate and the thermal figure is obvious; in the whole loading process the fracture starts from the local part, develops and evolutes gradually; with the stress change on the metal test-piece the infrared radiation tempreture of the test-piece surface changes, having the temperature-rising tendency in the total and the temperature rising trend in the local fracture position is obvious. This is because the metal test-piece material or the force is uneven and the stress concentration is formed, In the position of the stress concentration the deformation destruction firstly occurs. Because the energy is released in the process of deformation destruction, so the temperature change is obvious in the fracture position. After the experiment the obvious necking-down phenomena occurs in the test-piece. At the same time, in order to provide the theoretical basis for the experiment, with the finite element software of FEPG the finite element program is worked out, with the elastic-plastic model and the uneven material the fracture evolution process of the metal test-piece is simulated. Through the comparasion we can see that obvious necking-down phenomena appeared in the metal test-piece under the tensile load, the infrared information testing shows the temperature-rising is obvious in the position where necking-down phenomena occurs and the tensile necking-down phenomena of the metal test-piece is well simulated, which can provide helpful basis for the prediction and forecast of mental structure fracture.