The invention belongs to the field of pharmacy of traditional Chinese medicines and natural medicines and relates to a hydrangea leaf extract and medical composition, a preparation method and an application thereof. The invention aims to provide an application of total phenolic acid extract of hydrangea leaves in preparing a drug for treating thyroid cancer. The preparation method of the hydrangea leaf extract and medical composition comprises the following steps of: (1) taking hydrangea leaves, crushing the same and adding a solvent to soak and extract; (2) filtering the mixture, and concentrating the filtrate for later use; and (3) extracting the concentrated liquor to obtain extract, and adding water into the extract to filter to obtain the filtrate; (4) adsorbing the filtrate with a macroporous resin column, eluting the filtrate with distilled water, a 10-20% ethanol aqueous solution, 20-30% ethanol and 30-40% ethanol successively, collecting an 30-40% ethanol eluant, concentrating and drying the 30-40% ethanol eluant, crushing and sieving the same to obtain the hydrangea leaf extract. The total phenolic acid content in the total phenolic acid extract of hydrangea leaves reaches up to 40%, and the quality is stable and controllable. Through in vitro and vivo anti-thyroid cancer pharmacological tests, the extract has a clear treatment effect on thyroid cancer.