The microstructures and isothermal oxidation behaviors of the CoCrAlYTSi coating prepared by plasma spaying on the surface of the Ni-based superalloy GH202 were investigated. The cross-sectional sheet of the coating was prepared by focused ion beam (FIB) and the resulting microstructures were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). After oxidization at 800 °C to 1000 °C for 100 h, complex oxides were formed on the coating surface. The oxides included Al2O3, Cr2O3, CoO, CoCr2O4, Ta2O5, and SiO2. The protective oxidation films of Al2O3 formed readily on the coating due to the higher affinity of aluminum and oxygen. The addition of yttrium can increase the fineness of the coating microstructures and improve the adhesion of the coating. The oxidation of tantalum in the coating reduces the oxygen partial pressure in the oxidation scales and accelerates the formation of Al2O3. The addition of silicon element at less than 3 wt.% can slow the oxidation rate of the CoCrAlYTaSi coating, enhance the oxide scale adherence, and prevent outward diffusion of refractory elements (such as W and Mo) from the substrate. Overall, the presence of yttrium, tantalum, and silicon in the coating improved the resistance of high temperature oxidation and enhanced the bonding force of the coating and the alloy.