Background: Branched-chain amino acid (BCAA) is involved in glucose and lipid metabolism and energy homeostasis, but whether its effects on maternal metabolism and offspring growth during lactation depend on dietary protein level remains unclear. Methods: C57BL/6J lactating dams were randomized after delivery to a normal-protein control group (CN, 20% protein), low-protein group (LP, 10% protein), CN + BCAA group (CNB), or LP + BCAA group (LPB) (n = 11/group). Maternal body composition, glucose and lipid metabolism, hepatic energy status, oxidative stress indices, and pup litter weight gain were assessed. Results: The LP diet reduced maternal body weight, pup litter weight gain, and glucose responses during the intraperitoneal pyruvate tolerance test (IPPTT), and increased hepatic lipid deposition and oxidative stress. BCAA supplementation increased pup litter weight gain under normal-protein conditions, but decreased maternal food intake and offspring growth under low-protein conditions. Metabolically, BCAA supplementation reduced the homeostasis model assessment of insulin resistance (HOMA-IR) and hepatic PEPCK1 activity under normal-protein conditions, while partially restoring IPPTT glucose responses and increasing hepatic PEPCK1 activity under low-protein conditions. In addition, BCAA supplementation reduced hepatic lipid deposition and ATP content at both protein levels. Notably, under low-protein conditions, BCAA supplementation increased serum β-hydroxybutyrate (β-HB), while alleviating hepatic oxidative stress. Conclusions: BCAA supplementation exerted opposite effects on offspring growth during lactation under different maternal dietary protein levels. These effects may be related to differential changes in maternal glucose and lipid metabolism and hepatic energy status, suggesting that BCAA supplementation during lactation should be evaluated according to maternal dietary protein level.