A new signal model is built aiming to the problem of low-angle altitude measurement in complex environment, in which the multi-path submits to Gaussian distributed source. A generalized constrained MUltiple SIgnal Classification(MUSIC) algorithm is proposed, which takes the multi-path as constraint and improves direct signal precision by deflating the multi-path steering vector in signal subspace. The performance of the proposed algorithm would not be affected by coherent source and reflection coefficient. The proposed method achieves higher resolution under complex reflection surface in comparison with traditional high resolution methods. Simulation results and the real data processing demonstrate the validity and feasibility of the proposed approach.