The invention relates to a method for preparing a water soluble medicament-entrapping ultrasound contrast agent by the freeze-drying method. The method comprises the following steps of: dissolving water soluble medicaments in aqueous solution of which the mass is 5 to 50 times that of the freeze-drying powder of the lipid ultrasound contrast agent, adding the mixed solution into the blank freeze-drying powder of the lipid ultrasound contrast agent and mixing the materials for 5 to 30 minutes, adding a pharmaceutically acceptable surface active agent or stabilizer, of which the mass is 1 to 30 percent of that of the freeze-drying powder, into the mixed solution, uniformly mixing the mixed solution, adding the mixed solution into chitosan or alginic acid-containing aqueous solution, of which the volume is 1 to 5 times that of the mixed solution, and performing the freeze-drying treatment to form the water soluble medicament-entrapping ultrasound contrast agent. The method for preparing the water soluble medicament-entrapping ultrasound contrast agent by the blank contrast agent has the advantages of carrying out the medicament-entrapping process at the low temperature or normal temperature, introducing a few impurities, being easily operated and achieving a higher entrapment rate.