Radial countersunk screw lap joints widely employed to connect adjacent cabin sections of flight vehicle airframe. When subjected to flight load and thermal load, the joint stiffness exhibits complex nonlinear characteristics, affecting the dynamic response of the structure. In this work, experimental study is presented to understand the load-deformation behavior and the influence of thermal effect on the joint stiffness. The temperatures are controlled using a temperature box, and the experimental tests are conducted at room temperature, 50 °C, 100 °C, 150 °C and 200 °C. Axial tensile and compressive loads are applied using a MTS universal testing machine, and digital image correlation is employed to measure the deformation displacement. The results indicate that the load-deformation curve has obvious staged characteristics due to changes in the contact state between the components. Significantly, the thermal expansion and material soften caused by thermal effects lead to a decrease in joint stiffness. Compared to room temperature, the initial tension joint stiffness at 200 °C is decrease by 10.37%, and the initial compression joint stiffness decrease by 10.72%. These findings have reference significance for the structural design and optimization of flight vehicles.