Aiming at the problem of detection of range spread target by high-resolution radar in clutter environment, a range spread target detector λ-GLRT based on the estimation of auxiliary data matrix is proposed. The algorithm uses the known clutter covariance matrix to obtain the target detector. Combined with the auxiliary data estimation clutter covariance matrix, the estimation matrix is substituted into the detector, so as to realize the purpose of clutter suppression and target detection. The simulation results show that the proposed detector has better target detection performance than GMF and NMFI detector, which is adaptive to clutter.