The present invention relates to: a thermosetting alkoxysilyl compound (hereinafter, referred to as “alkoxysilyl compound”) having two or more alkoxysilyl groups showing excellent heat-resistance characteristics in a composite; a composition and a cured product comprising the same; a use thereof; and a method for preparing an alkoxysilyl compound. The composition of an alkoxysilyl compound, comprising a novel alkoxysilyl compound according to the present invention shows, in a composite, improved heat-resistance characteristics, i.e., an effect of decreasing the CTE of the composition of an alkoxysilyl compound and not showing a glass transition temperature (hereinafter, referred to as “Tg-less”). Further, the cured product comprising an alkoxysilyl compound according to the present invention shows excellent flame retardant properties due to the alkoxysilyl groups.