Sorption isotherms of p-NCB on black soil at different time (1 day and 36 days) were overlapped, suggesting the apparent sorption equilibrium was achieved in 1 day. But the desorption isotherms under the same condition were considerable different. Maybe after sorption achieving equilibrium, the interactions which were similar to “induced-fit” effect in biological enzyme catalytic reaction between p-NCB molecules and the soil organic matter were became much stronger as the reaction time prolonged. Sorption hysteresis was observed in high concentration range, the soil organic matter might be deformed by the high concentration sorbate.