As numerous studies show, the number of children with speech disorders is increasing every year. Among preschoolers with speech disorders, children with general speech underdevelopment constitute the largest group - about 40%. Despite the fact that the theoretical foundations of the formation of grammatical structures in the speech of children with speech disorders have been determined, the specificity of their assimilation of prepositions with spatial meaning has not been sufficiently studied and the methods of formation of prepositional-case forms, caused by the underdevelopment of optical-spatial representations, have been determined. To study the level of formation of spatial gnosis, understanding and possession of prepositional - case constructions in older children of preschool children with speech underdevelopment, a special study was carried out. The revealed specific violations in the assimilation of the morphological and syntactic system in preschoolers with general speech underdevelopment allowed us to conclude that it was necessary to carry out a special corrective action on the formation and differentiation of prepositional-case structures in this category of children. After one school year, in the control experiment, the children were offered the same tasks as at the beginning of the school year. The obtained results of the study of optical-spatial representations and skills of constructing prepositional-case structures before and after carrying out correctional work using the author's methodology in preschoolers with general speech underdevelopment of the III level made it possible to conclude that purposeful correctional and speech therapy work on the formation of spatial gnosis, understanding and possession of prepositional-case constructions contributes not only to the development of this mental process and associated speech processes, but to the development of speech in general and the cognitive activity of children.