Changes in compost components produced over different composting periods and with addition of 0, 5, 10, and 15% humic acids (HA) were analyzed by near-infrared spectroscopy (NIRS) in order to investigate the transformation mechanism of green (plant) waste into compost. The results indicated that HA addition can not only accelerate the composting process but also improve the compost quality by increasing its maturity. HA addition does not change the functional groups but does change their number and the structure of the materials. Addition of 10% HA was found to have the most beneficial effects on the composting process and on the resulting compost.