The invention relates to a method for preparing a folate molecular targeted magnetic nanometer medicine carrier, which comprises the following steps of: 1) preparing solution of aldehyde of sodium alginate (ASA), namely dissolving sodium alginate (SA) in water, adding a strong oxidant for reacting for 24 to 36 hours, and adding ethanol to stop a reaction; 2) preparing solution of magnetic nanometer particles (MNPs), namely dissolving a ferrous salt and a ferric salt in the water, regulating the pH value to be 9 to 10, stirring until the color of the solution is dark black, and extracting supernate for later use; 3) preparing aldehyde of sodium alginate modified magnetic nanometer particles (ASAMNPs), namely mixing the ASA solution in the step 1 and the MNPs solution in the step 2, reacting at the temperature of between 80 and 90 DEG C for 30 to 50 minutes, and taking supernate for later use; 4) preparing folic acid-amino polyethylene glycol (FA-PEG-NH2); and 5) preparing FA-PEG-ASAMNPs. The prepared FA-PEG-ASAMNPs serving as a carrier have the high performance of responding to tumor cell, and the magnetic particles are connected with the carrier, so the carrier has the higher magnetic response performance.