A new dimeric eudesmane sesquiterpenoid with an unreported 6/6/5/6/6 fused carbon skeleton, echingridimer A (1), and five monomers (2-6), including two new compounds (2 and 3), were isolated from the roots of Echinops grijsii. Their structures and absolute configurations were elucidated by comprehensive spectroscopic analyses and confirmed by X-ray crystallography or electronic circular dichroism spectra. Compound 1 represents the first example of a 3,15'-linked Michael-type adduct of two eudesmanes, which feature a unique 1-oxaspiro[4.5]decan moiety. The plausible biogenetic pathway for 1-6 was briefly discussed. The oxaspiro dimer (1) not only showed stronger aphicidal activity than closely related monomeric compounds (2-6) and the commercial aphicide pymetrozine but also exhibited remarkable insecticide activity against a broad range of aphids, thus could be used as a potential biobased insecticide to suppress aphids.