Combined with one specific engineering example, in which the novelty and complexity of the bridge has to be taken into account, a complex spatial simulation model including beam, plate, and solid elements is established by the application of MIDAS/Civil2006, then its internal force is calculated and analyzed. But through this method, the model is complex and the calculation efficiency is not high. So how to use the grillage theory to construct model for the bridge to improve the design efficiency is necessary. Specifically, the model is established only by beam elements, in which compression-flexure member, shear-bending member is simplified as grillage or column case. The result of the comparative analysis on the two models shows that both the support reaction and the internal force or stress are in agreement under the dead weight and moving load, as well as the fundamental frequency and vibration. So the result of the simplified model according grillage for unitary analysis has sufficient precision. And the simplified method can provide some reference for spatial analysis of other similar bridges.