Abstract: In order to investigate the dyeing mechanism of lac pigment on nylon fiber and providing theoretical basis of the dyeing process optimization, the dyeing equilibrium experiments of lac dye on nylon fiber were carried out. Freundlich, Langmuir and Redich-Peterson isotherms were used to simulate the experimental data at 80 and 90℃， some thermodynamics parameters such as dyeing affinity(-Δμ°), enthalpy change(ΔHo) and entropy change(ΔSo) were also evaluated. The results indicated that the adsorption process of lac pigment onto nylon fiber fits Redlich-Peterson model best, and the dyeing process is an exothermal and spontaneous one, the values of enthalpy change (ΔHo) and entropy change (ΔSo) were -55.45kJ/mol and -111.96J/mol respectively. In addition, the dyeing process affinity and adsorption quantity decreased as temperature increased.