The lignin from moso bamboo was fractionated mildly and efficiently using formic acid / guanidine hydrochloride acidic deep eutectic solvent (DES). The structure, thermal stability and antioxidant activity of the lignin were characterized. It was found that formic acid / guanidine hydrochloride (molar ratio was 4:1) DES resulted in a high lignin yield of 59.31% under mild conditions (100 ℃, 2 h) and further increased to 73.98% at 120 ℃. This was attributed to the acidic DES donated active protons to selectively cleave β-O-4、β-β、β-5 bonds and further facilitated a large amount of lignin removal. The following lignin profiling indicated that the lignin had a high purity (>96.6%), low molecular weight (Mw=1040~2040 g/mol), low polydispersity (PDI<1.5), homogeneous structure, excellent thermal stability. The antioxidant performance of the lignin (IC50=0.016~0.045 mg/ml) was better than that of the commercial antioxidant butylated hydroxytoluence (IC50=0.056 mg/ml).