Based on the comparative test data from February to July in 2005, the difference of air temperature / relative humidity measured inside the thermometer screen of glass fiber reinforced plastic and that of timber are analyzed. In the meantime, the difference of response speed to air temperature as well as relative humidity changes in the two kinds of screens is analyzed. The impact of various cloud cover and various wind velocity on the difference of air temperature measured inside the two screens is discussed. The result indicates that the mean difference of air temperature inside the two kinds of screens is within 0.1℃, the standard deviation of the difference is within 0.2℃. The mean difference of relative humidity inside the two kinds of screens is within 0.4%, the standard deviation of the difference is within 2.1%. The response to air temperature / relative humidity inside the thermometer screen of glass fiber reinforced plastic is almost faster than that in the screen of timber. No matter under what cloud cover or what wind velocity, the mean difference of air temperature in the two kinds of screens is almost within 0.1℃.