In Gram-negative bacteria, assembly of the beta-barrel outer membrane relies on the beta-barrel assembly machinery (BAM) complex. This highly sophisticated nanomachine integrates and assembles proteins into the outer membrane and is composed of five subunits: the integral membrane protein BamA, and four lipoproteins called BamB, BamC, BamD and BamE. Recently, crystal structures were reported for the BamABCDE complex, as well as a form lacking BamB, the BamACDE complex. While these structural snapshots provide crucial clues about how the BAM complex functions, substrate protein pathway in the BAM complex and dynamics of the subunit-subunit interaction during substrate protein translocating is still unrevealed. We employed the in vivo site-specific photo-crosslinking method within BamA to map the interactions of BamA with the other BAM accessory proteins. The majority of crosslinks observed corresponded to interactions shown in previous crystal structures of the BAM complex. However when probing for interactions within the extracellular loop of BamA we observed novel crosslinks to BamC. Moreover chemical amount of substrate protein adding induced the changing of amount of the crosslink products and the crosslinked amino acid position of the BamA. These results proposed that the mechanical dynamics of the BAM complex is required for protein assembly in the non-energy supplied environment.