Introduction: Bmal1 is an important circadian rhythm gene, and its effect on sepsis has not been entirely elucidated. The aim of this work was to explore the regulatory role and molecular mechanism of BMAL1 in sepsis. Methods: C57BL/6J mice and THP-1 macrophages were used to establish sepsis models in vivo and in vitro, respectively. The peritoneal macrophages (PMs) and liver Kupffer cells (KCs) of mice were extracted, and the expressions of BMAL1 and PGC-1α in various macrophages were detected by Western blotting and RT-qPCR. Overexpression of BMAL1 in macrophages with plasmid. Knocked down the expression of PGC-1α by short hairpin RNAs (shRNA). STL1267 is an inhibitor of BMAL1, was used to inhibit the expression of BMAL1 in mice. Western blotting was used to detected the expression of BMAL1 in macrophages. ELISA was used to detected the levels of imerleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) in cell supernatant and mouse serum, and HE staining was used to detected the pathological changes of organs. In addition, we detected the mutual regulation of BMAL1 and PGC-1α. Results: Both the sepsis models in vivo and in vitro, the protein and mRNA expression of BMAL1 in macrophages were significantly decreased. Overexpression of BMAL1 inhibits LPS-mediated inflammatory response and promotes M2-type polarization of macrophages. Inhibition the expression of BMAL1 by STL1267 further aggravated the inflammatory reaction and multiple organ damage in septic mice. PGC-1α is the downstream factor of BMAL1. After silencing PGC-1α, the inhibitory effect of BMAL1 on inflammatory response of macrophages was significantly weakened. Conclusions: BMAL1 is a key factor in inhibiting macrophage inflammatory response and sepsis, and its molecular mechanism closely related to the regulation of BMAL1 to PGC-1α.