The present invention relates to a use of total coumarins of cnidium fruit in preparation of a medicament for treating psoriasis. The invention can significantly inhibit mitosis of vaginal epithelial cells of rats； promote the formation of epidermal granular stratum in rat tail's scale； inhibit passive cutaneous anaphylaxis of rats； significantly reduce ear-swelling of mouse induced by croton oil and foot-swelling of rats induced by egg-white； and lessen the itching reaction caused by increasing itch-causing threshold of guinea pigs caused by histamine phosphate. The total coumarins of the invention can improve the clinical symptom of psoriasis, have higher curative effect and are safe to use. It can be administrated both externally and orally.
