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Typical seismic sway braces (SSBs) are composed of several bolted steel components. As the connecting bolts become loose, the stiffness and bearing capacity of SSBs decline from which their seismic performance degrades significantly. Based on the vibration response of the brace components, a framework of a vibration-based pre-alarming method for the bolt looseness of SSBs is created and established. Four damage indexes were constructed based on the methods of Fast Fourier Transform (FFT), Wavelet Packet (WP), Intrinsic Mode Function (IMF) and Hilbert-Huang Transform (HHT). Having tested and verified their sensitivity to the bolting torque of bolts, a multi-index hierarchical early warning system (MHES) was established by associating the four damage indexes with the tightening torque of the bolts, by which the threshold of the yellow (slightly damaged) and red (severely damaged) warnings were defined and confirmed. By means of the application in an actual project, the validation of the MHES was finally verified and confirmed according to its stability and accuracy.