We investigated the parasitism rate,dry weight of shoot and root,plant height of different Helianthus annuus varieties in seedling stage,measured the content of malondialdehyde (MDA) and the activities of enzyme PAL,SOD,POD and CAT in leaves after sowing 30,50 and 70 days,respectively.The seeds were sown in plastic pots and the soil was collected from continuous H.annuus cropping yield where the Orobanche cumana infection was severe.The results indicated that different H.annuus varieties in seedling stage exhibited obviously different resistance to O.cumana.Oil H.annuus varieties ‘T012244’ and ‘MGS’ were immune to O.cumana in seedling stage,‘S31’ and ‘Baiza 9’ showed a strong resistance to O.cumana.However,‘Xinghuodabaibian’ and ‘Baiza 6’ were sensitive to O.cumana in their seedling stage.Parasitization of O.cumana could restrain the growth of sensitive H.annuus varieties but hardly impact the growth of resistant H.annuus varieties.After parasitization of O.cumana,the MDA content and the PAL activity in leaves of ‘Xinghuodabaibian’ seedling increased significantly,which indicated an aggravated damage on H.annuus seedings.However,the MDA content in leaves of varieties‘T012244’and‘S31’ decreased and the activity of PAL did not significantly changed.The enzyme activities of SOD,POD and CAT in different H.annuus varieties all decreased after an initial increase.The change ranges of protective enzyme activities in immune varieties are bigger than that in sensitive varieties.The parasitization of O.cumana put the sensitive H.annuus varieties under environmental stress in seedling stage,defense reactions based on protective enzyme system were induced in seedling leaves.