Bi-linear reduced-order modeling (BLM) is an effective method for vibration problem of elastic structures with intermittent contact nonlinearity (e.g. the simplest case is a plate with a crack). The rationale at the basis of this technique is that, in a given frequency range of interest, displacement of structure can be approximated by linear superposition of two sets of linear constraint modes of specific contact boundary conditions (BCs) (that is, a specific part of contact surface is closed and the rest is open, and the opposite situation). Contact BCs of the two sets of modes have essential effect on the accuracy of this method. Previous bi-linear reduced-order method assumes that BCs of contact surfaces are always the same, but in fact they would changes with frequency of excitation. The goal of this research is to set up a new adaptive contact surface calculating method for bi-linear reduced-order modeling which can calculate and update contact BCs of structure at any specific frequency. Our research may improve the accuracy of bi-linear reduced-order modeling method to a considerable extent.