Interaction of high-power laser with planar target is studied by using radiation-hydrodynamics simulation. When the laser interacts with the tungsten planar target, the laser energy deposition is uneven due to thermal filamentation instability and other reasons, and density fluctuations will appear in the front of the plasma, resulting in obvious plasma filamentation in the later stage. The researches of four materials, i.e. aluminum, copper, tungsten and gold, show that in the high-Z material tungsten and gold, due to the strong radiative cooling effect, the filamentation phenomena of the density distribution, electron temperature distribution and pressure distribution obviously occur. The order of magnitude of filamentous plasma density is different from that of the surrounding plasma. The filamentation phenomenon is closely related to the non-uniform energy deposition of the laser and the radiative cooling effect, although the ray beam will cause inhomogeneity of the laser irradiation to a certain extent, this is not the main reason for the filamentation phenomenon observed in this paper. Owing to refraction, reflection and the thermal filamentation instability when the laser is transmitted in the ablation plasma, the laser energy is deposited unevenly, which generates instability seeds in the early stage of plasma formation. The radiative cooling effect then amplifies this instability seeds, creating a radiative cooling instability that eventually results in a filamentous distribution of physical quantities such as plasma density, temperature, and pressure. This filamentation phenomenon destroys the uniformity of the plasma to a certain extent, and lays the seeds for the growth of fluid instability, which will seriously affect fusion-related research. It is shown that radiative cooling is crucial to the filamentation phenomenon, which causes uneven distribution of the plasma pressure during the evolution of the plasma, thereby affecting its transverse motion and enhancing the density fluctuation. After the laser irradiation ends, the density fluctuation gradually develops into filamentations. We also find that the clear filamentation occurs only for high-Z materials like tungsten and gold, but not for the moderate-Z materials like aluminum and copper. This can be attributed to the fact that radiative cooling is stronger for the high-Z materials. Studying the filamentation effect in laser-irradiated planar targets can contribute to understanding the instability in laser plasma, and then suppressing this instability and improving the gain of fusion. The results here can thus be of reference significance to the research of laser fusion, laboratory astrophysics, and other applications of intense-laserdriving.