Considering the adaptive target detection of radar in heterogeneous environments, we proposed a knowledge-aided method for clutter covariance matrix estimation.The clutter in heterogeneous environments was supposed to subject to complex Gaussian distribution with covariance matrix as the random covariance matrix.The inverse Wishart distribution was employed as the priori distribution.According to Bayesian methods, we derived the posteriori probability density function of clutter covariance matrix.Then the maximum a posteriori estimation of covariance matrix was obtained.The simulation results show that the proposed knowledge-aided estimation method can effectively improve the target detection performance of radar under heterogeneous environments.