When the real vegetation is composed of three layers, the accuracy of the traditional three-stage vegetation parameter inversion method becomes lower. To solve this problem, a new polarimetric SAR interferometric coherence model was established. Based on this new model, the reason why the accuracy of the tranditional three-stage vegetation parameter inversion method becomes lower was analyzed and a new vegetation parameter inversion method based on dual-baseline polarimetric SAR interferometry was proposed. Compared to the tranditional inversion method, this new method is more suitable to the three-layer vegetation constructions and has higher accuracy. Experimental results validated the correctness and the superiority of the new method.