The theory modal analysis (TMA,forward problem) and experimental modal analysis (EMA,inverse problem) methods were both used to analyze the dynamics characteristic parameters of one commercial vehicle cab's body-in-white. Finite element modal analysis was carried out to obtain the mode shape and lay down the basis for experimental modal analysis in TMA process. In EMA process, LSCE(Least Squares Complex exponent method) and LSCF (Least Squares Complex Frequency Domain method, commercial name is PolyMAX) methods were used to get the mode shape and modify the TMA results. Based on the comparison among all the results, the accurate conclusion can be reached and the improvement measures can be brought forward to the prototype. The improvement projection was proved to be effective by consumers' utilization after manufacturer put it into applications.