Parasitic plants acquire water and nutrients from their hosts through haustoria, unique organs that function in host invasion and nutrient uptake. The formation of prehaustoria, the preliminary structures of haustoria, is induced by host-derived small compounds called Haustorium Inducing Factors (HIFs), which are originated from the cell wall lignin biosynthesis pathway, at least partially. Since lignin is conserved almost all vascular plants, prehaustorium induction occurs against a wide range of plant species. However, the parasitic plants in the Orobanchaceae rarely form prehaustoria against themselves or parasitic species in the same family; this phenomenon is called “avoidance of self (or kin) parasitism”. To understand how parasitic plants avoid kin species, we isolated a mutant that forms spontaneous prehaustoria, named spontaneous prehaustorium 1 (spoh1), from the model parasitic plant Phtheirospermum japonicum. spoh1 forms prehaustoria in the absence of host or HIFs, while wild type does not. Addition of sucrose, but not other sugars, can enhance the spontaneous prehaustorium formation in spoh1. Root exudates of spoh1 incubated in sucrose solution can induce prehaustorium formation of Striga hermonthica, a related parasitic species. Thus, SPOH1 is required to suppress the exudation of HIFs from P. japonicum roots. Whole genome sequencing identified the responsible mutation in a gene encoding a UDP-glucose-dependent glucosyltransferase. E.coli-expressed SPOH1 can glucosylate an HIF in vitro, suggesting that SPOH1 glucosylates endogenous HIFs in parasitic plants and thus inactivates them. We propose a novel mechanism for the avoidance of self- or kin- parasitism by modifying plant cell wall components.