To　improve　the　mode　decomposition　capacity　for　underdetermined　and　unclear　modes,　a　modified　blind　source　separation　(MBSS)　method　has　been　proposed,　where　a　multi-synchroextracting　transform　algorithm　with　a　sliding　window　is　proposed　for　a　higher　sparsity　time-frequency　spectrum.　The　proposed　transform　algorithm　incorporates　an　iterative　formula　of　the　instantaneous　frequency　with　a　sliding　window.　Then,　it　is　embedded　into　the　existing　novel　blind　source　separation　(NBSS)　method　to　highly　improve　the　modal　decomposition　accuracy.　The　feasibility　of　the　proposed　method　has　been　verified　against　a　numerical　3DOF　mass-spring-damper　system,　a　numerical　three-story　frame　structure,　and　an　experimental　five-story　steel　frame.　The　analysis　results　demonstrate　that　the　proposed　MBSS　method　can　well　decompose　the　acceleration　signals,　providing　better　precisions　than　the　NBSS　method　under　the　circumstance　of　unclear　and　underdetermined　modes.　Moreover,　the　proposed　method　has　higher　decomposition　accuracy　for　close　modes.