Abstract: The paper calculates and analyses vertical distribution of heat exchange and its change with depth and time,The amplitude of heat exchange in various layer of soil increases with depth,but its value decreases in index law,and is approximately proportional to average amplitude of temperalude gradient in the same layer of soil;The phase of heat exchange delays with depth in accordance with linearity,but the initial phase is earlier π/4 than average temperature wave in the same layer,On this basis of calculation of heat exchange,using Lagrange interpolation,the paper calculates the vertical distribution of heat flux of soil layer from surface to depth of 3cm and indicates that in order to measure heat flux of surface soil,it is better to put the transducer of flux plate under the surface soil about 0.5cm of depth.