Objective To observe the therapeutic effect of external application of herbal medicine for ankylosing spondylitis(AS).Methods Two hundred and seventy-nine AS patients were randomized into group A(N=175)and group B(N=104).The two groups received oral use of sulfasalazine,and group A additionally received external application of herbal medical ointment(mainly composed of Pheretima,Squama Manis,Rhizoma Dioscoreae Nipponicae,Radix Rehmanniae Preparata,Rhizoma Cibotii,Cortex Eucommiae,Radix Dipsaci)on 3~4 pairs of bilateral acupoints of Jiaji,dressing change once every three days,for continuous 6 months.After treatment,the therapeutic effect was evaluated.The primary outcome measures of health general score(BAS-G),disease activity index(BASDAI),function index(BASFI),Likert four-grade scores of general pain and rachialgia,duration of morning stiffness,general assessment by physician were observed before and after treatment.The secondary outcome measures of occiput to wall distance,finger to floor distance,chest expansion,Schober's test,spinal movement,"4"shape test were compared.The changes of erythrocyte sedimentation rate(ESR),C-reactive protein(CRP),complements 3 and 4(C3 and C4)as well as the adverse actions were also observed.Results After treatment,46 patients were clinically relieved,65 markedly effective,52 effective,12 ineffective,and the total effective rate was 93.14% in group A;11 patients were clinically relieved,27 markedly effective,48 effective,18 ineffective,and the total effective rate was 82.61% in group B.The difference of the therapeutic effect was significant between the two groups(P