Boundary conditions are foundation of structural modal analysis. Different boundary conditions loading methods influence structural modal parameters via changing the structural stiffness. By comparing the basic principles of computational modal analysis and experimental modal analysis through theoretical deduction, and establishing five kinds of feasible boundary conditions for proportion body finite element model, maximum error of elastic suspension of top 30 step modal parameters of proportion body compared to free mode was calculated to be 0.02346%, and the maximum error of boundary conditions 4 and 5 was 25.3386% and 10.2383%, respectively. Therefore, we could utilize elastic suspension simulated structure free mode in experimental modal. With analysis of hammering modal test for proportion body while three different boundary conditions respectively effecting (spring suspension, spring rod suspension and air spring support), the maximum error of first 3 vertical bending frequencies was obtained to be 2.545%, 2.961% and 4.812%, respectively, less than the error tolerance(5%). The correlation of natural frequency of structure in test mode was also obtained through mode assurance criterion(MAC matrix), which demonstrated the effect of boundary conditions on the modal parameters of proportion body.