Abstract: An engineering model for hypersonic reentry body trail is established in this paper,and a calculation is performed by using boundary layer theory.Prandtl's mixture length theory of turbulent shearing stress and Taylor's eddy exchange theory are used to predict the distribution of velocity and temperature respectively.Lykoudis's method is used to calculate the parameters at the axis of the trail.Also the trail length and free electron number density of the trail are predicted.The results are in agreement with those of the relative references.The purpose of this paper is to give those parameters required for the computation of the radar diffusion section of the trail.