In a nuclear power plant, temperature fluctuations of non-isothermal turbulent jets may cause fatigue damage to the surfaces of the upper plenum structure of a nuclear reactor, which is known as thermal striping. In the past few decades, various thermal striping studies on the upper plenum of nuclear reactors have been carried out. Different fluid jet models such as the parallel jet model, the triple parallel jet model and the coaxial jet model have been used to study the origin and the possible avoiding of these temperature fluctuations. However, most of these studies published so far are numerical simulations, only few experimental data are available. We have carried out both a numerical simulation to model temperature fluctuations in a fluid and a solid plate under non-isothermal coaxial jets using a simple domain geometry as well as experimental studies. We conducted the experimental investigation using hot and cold coaxial water jets at East China University of Science and Technology to validate the numerical results. The instantaneous temperature fluctuations in the fluid and on the plate surface were measured locally. In the present work, we develop a numerical model with the solver chtMultiRegionFoam within the frame of the open course platform 'OpenFOAM'. The computational domain is set according to the real test segment of the test rig. The LES turbulent model is tested and compared with the experimental data. It is found that the mean temperature distributions in the fluid and plate is slightly axisymmetric, and that the variance distribution along the radius direction has a peak. The amplitude-frequency-diagram and temperature fluctuations obtained by experiment and numerical simulation are analyzed and discussed. These results can be later used further for the calculation of the thermal stress in the plate, so that the fatigue life of the plate structure due to thermal striping can be evaluated.