Abstract: The allelopathy and its physiobiochemical mechanism of aqueous cucumber aboveground part extract on tomato were investigated under culture experiment. Chemical constituents of the extract were qualitatively analyzed using the test-tube method. The results show that cucumber aboveground part extract contains akaloids, phenols, organic acids, saponinsand, sterols, polysaccharides, protein and fats oils, but not tannins. The aqueous extract significantly inhibits the respiration rate and α-amylase activity during tomato seed germination. It also inhibits germination rate, germination index and the inhibition effect increase with increasing concentration of the aqueous extract. Watering tomato seedlings with the aqueous extract increases the contents of O₂^-., H₂O₂, glutathione (GSH) and ascorbic acid (ASA), malondialdehyde (MDA) and plasma membrane permeability in leaves of tomato seedlings and decreases catelase (CAT) and peroxidase (POD) activity. Superoxide dismutase (SOD) and ascorbate peroxidase (APX) activity initially increases and then decreases with increasing concentration of the aqueous extract. Also chlorophyll content, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and dry seedling weight decrease while intercellular carbon dioxide content (Ci) increases. Kinetic data on chlorophyll fluorescence reveal that the minimal initial fluorescence (Fo) of tomato seedling leaves increases with increasing aqueous extract concentration. The maximum fluorescence (Fm), PSII potential fluorescence efficiency (Fv/Fo), PSII intrinsic photochemical efficiency (Fv/Fm), actual photochemical efficiency of PSII (ΦPSII), photochemical quenching (qP), electron transport rate (ETR) and photochemical rate (PCRNPQ) initially increases and then decreases, which is harmful to the photosynthetic structure.