A second-order closure model was developed to predict the thermal dispersion problem behind a line-heat-source in turbulent flow. The exact governing equations of the Reynolds stress, heat flux and other sealer fluctuating quantities were formulated and modeled. Numerical computations were carried out for two kinds of thermal turbulent flows. The distributions of mean and fluctuating quantities of velocity and temperature at different locations downstream of the line-heat-source were obtained and compared with experimental results.