In the experiment of measuring the thermal conductivity by steady-state method，the tested plate cooled in the air is usually put on the cooling plate to measure the heat dissipation rate at the stable temperature．We simulate the natural cooling process of the tested plate，which is placed on the cooling plate at different times． The difference of the internal temperature field affects the thermal conductivity． The linear relationship between the cooling time of the tested plate and the thermal conductivity is studied． Under the condition of constant temperature of the tested plate，the corresponding thermal conductivity coefficients at

different temperatures are given，which indicates that the temperature field distribution in the tested plate significantly affects the measurement of thermal conductivity．The theoretical optimal cooling time and constant temperature value are given under the room temperature of 20 ℃，which can provide reference for experimental research．