A numerical simulation study is conducted on SiC_f/SiC ceramic matrix composite turbine guide vanes to investigate the effects of variations in temperature ratio and flow ratio on the overall temperature and cooling performance of the vanes. A total of 16 operating conditions with variable temperature ratios and flow ratios are selected for simulation calculations. The results show that with the increase of both flow ratio and temperature ratio, the maximum temperature, minimum temperature and average temperature of the vanes all decrease, and high temperature occurs at the leading edge of the lower platform. The cooling efficiency of the vane suction surface increases gradually from the leading edge to the trailing edge, while that of the vane pressure surface first decreases and then increases from the leading edge to the trailing edge. For the local cooling efficiency, when the flow ratios are 5.28%, 7.54%, and 8.44%, respectively, the average cooling efficiency first decreases and then increases with the rise of temperature ratio. When the flow ratio is 3.69%, the average cooling efficiency decreases gradually as the temperature ratio increases. Meanwhile, the average cooling efficiency of the mid-section increases with the rise of flow ratio.