Three basic types of similar relationship between the prototype and the model for dynamic structural model test and dynamic destructive model test were proposed in corresponding literatures. At the time the situation where various similar relationships are applicable and the technique to ensure similarity for the different goal was discussed. Here the numerical simulation of model test of water-conveyance tunnel concerning fluid-structure interaction in soft soil is studied. Based on economy and practicability of selective material for model test, the similar relationship and the technique are proposed, which are validated through the example. The results of numerical simulation show: under the specific conditions, data of the model test can completely transfer to those of the prototype by use of this type of similar skill, and get more useful information. Some new ideas are introduced to keep the similarity of the hydro-structure structures.