The invention relates to the new dosage form of zanamivir, which is a nasal in-situ gel with phase transformation property and preparation methods, the invention is made of the original drug of zanamivir, hydrophilic gel materials which are sensitive to environments and assistant materials which can be accepted in pharmacy, after being absorbed by nasal mucosa, the invention can have systemic functions and improve the partial concentration of drugs in respiratory tracts, when room temperature is about 20 DEG C and the invention is in storing state, the invention is in the state of liquid, while after being put into the physiological conditions of nasal cavities, the invention can be quickly formed into gel on the surface of nasal mucosa, so that the detained time of drugs can be prolonged,biological availability can be improved and the compliance of patients can be perfected, and the viscosity of the invention is proper, after being formed into gel, the invention can be detained on the surface of the nasal cavity for a rather long time, which has no nasal ciliary toxicity.