In view of the different characteristics of modes between long and short strap-on launch vehicles, the differences and relations of elastic vibration modeling method of them were analyzed firstly. Then a new attitude dynamic model of solid strap-on launch vehicle was derived, the elastic vibration equation in the model was also derived by finite element method. Finally, the coupling among three channels based on the new model was studied. Inverse Nyquist Array method was used to design the controller. The results show that the new model can reflect more exactly the coupling characteristics of longitudinal, lateral and torsinal vibration of strap-on launch vehicle. Although the coupling among three channels exists, the coupling matrix has the diagonal dominance characteristics and the controller designed by Inverse Nyquist Array method can obtain a good performance.