The invention discloses a water-soluble tetravalent platinum compound and a preparing method and application thereof. The water-soluble tetravalent platinum compound is hydrated dyhydroxyl hydroxide tetraammineplatinum (IV) with the molecular formula of [Pt(NH3)4(OH)2](OH)2 and the structure seen in the description. The preparing method includes the steps of firstly, synthesizing a mixture of tetraammineplatinum chloride and ammonium chloride from platinum, wherein the mixture is directly used as a reaction raw material in the second step without being extracted; secondly, introducing CO2 intoa mixture solution of tetraammineplatinum chloride and ammonium chloride to form a CO2 saturated solution, filtering obtained precipitates, washing a filter cake with water four times, and conductingmixing to form slurry; thirdly, adding a certain amount of Ba(OH)2 or Ca(OH)2 to the slurry mixture, conducting dilute sulphuric acid detection until no Ba2+ or Ca2+ exists, conducting filtering, adding excessive hydrogen peroxide, and conducting boiling until granular crystals are separated out to obtain a target compound. The method is convenient to operate and suitable for industrial production.