Relying on the experimental data, by using the theory of error control and function approximation to establish new wet and dry bulb coefficientÂ(v, t_a). Through searching method and Iterative approximation, it finds an approximation function whose figure is closer to "experimental data" than previous one did. Hence, it increases the measurement accuracy of relative humidity by the wet and dry bulb method under the condition of the relative humidity in the range of 10℃≤t≤70℃, air flow rate 0.1m/s≤v≤4m/s, and relative humidity 30%≤U≤90%.