Stability constant logK, which tested three kinds of soil soluble humic acids reacted with copper ion to form the complexes, appeared cultivated loessal soil>old manured loessal soil>yellow Ioessal soil. logK value was related to pH, temperature and ionic strength. The thermodynamic function variations in complexation were: △rGm^θ<0, △rHm^θ<0, and △rSm^θ<0, showing that the complexation reaction was a spontaneous exothermic reaction and that negative value of △rSm^θ<0 was in the same sequence with logK, wich was considered as the entropy effect of humic acid molecular deformation and complex stability in process of humic acid reaction with copper ion.