Amidated humic acid (AHA) was prepared by amidation of humic acid (HA) and its structure was verified by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Then poly(lactic acid)/amidated humic acid composites (PLA/AHA) were prepared using AHA as the nucleating agent. The mechanical properties, thermal stability, and crystallization behavior of the PLA/AHA composites were investigated by thermogravimetric analysis (TG), differential scanning calorimetry (DSC), polarized optical microscopy (POM), and rheological analysis. The mechanical properties and thermogravimetric analysis showed that the tensile properties, impact resistance, and thermal stability of the PLA/AHA composites were improved. The cross-sectional morphology of the fractured surface showed that the addition of the nucleating agent significantly changed the crystallinity of PLA. The DSC results showed that the crystallinity of PLA/AHA composites increased from 4.19 to 35.36% after introduction of the nucleating agent. The POM of PLA/AHA composites demonstrated that AHA increased the crystallization rate of PLA. The maximum torque of PLA/AHA composites was 75.0% higher than that of PLA and the apparent viscosity was increased by 172.5%. This proved that the AHA changed the crystalline lattice structure and stiffness of PLA matrix. Non-isothermal crystallization kinetics showed that HA provided a large number of heterogeneous nucleation sites and improved the crystallization rate of PLA.