In order to study the allelopathy effect on seed germination and seedling growth of the aqueous extracts from leaves and leafstalks of Oxalis corymbosa, probing the invasive mechanism of alien species, the aqueous extracts of O. corymbosa were used as the test material in this paper. Its purpose was to study the influence of the aqueous extracts of O. corymbosa to Tagetes patula, Calendula officinalis and Celosia cristata three herbaceous flowers seed germination and seedling growth. The concentration of the aqueous extracts from leaves and leafstalks of O. corymbosa was 0 (CK), 0.625%, 1.25%, 2.5%. The results showed that sensitivity to the aqueous leaf extract of O. corymbosa differed among the three species. Concentration at 0.625% of the O. corymbosa aqueous extracts from leaves and leafstalks promoted the germination and seedling growth of Tagetes patula, however, concentration at 1.25%, 2.5% inhibited the germination and seedling growth of Tagetes patula. Concentration at 0.625%, 1.25%, 2.5% of the O. corymbosa aqueous extracts from leaves and leafstalks inhibited the germination and seedling growth of Calendula officinalis and Celosia cristata, but the inhibition of concentration gradient was not remarkable. The test showed the allelopathy of O. corymbosa to Tagetes patula was stronger, the allelopathy to Calendula officinalis and Celosia cristata was weaker.