The invention provides a synthesis method for an amphoteric ion copolymer for a well drilling fluid, wherein the synthesis method comprises the steps: weighing dimethylaminoethyl methacrylate and 1,3-propanesultone with the mass ratio of 2:1 to 9:1, adding 1,3-propanesultone to dimethylaminoethyl methacrylate, after reaction, filtering, extracting, and drying to obtain DMAPS; adding the obtained DMAPS and AM into a solvent, dissolving, then transferring into a reactor, next introducing N2, carrying out a deoxidization reaction, at the same time, heating up to 50-70 DEG C, then adding an initiator, carrying out a reaction to obtain a colloidal solid, and precipitating with acetone to obtain a white precipitate; and drying the obtained precipitate, and then crushing to finally obtain the amphoteric ion copolymer. The copolymer obtained by the method can sustain a temperature as high as 150 DEG C, resists NaCl saturation, resists CaCl2 saturation, has the filtrate loss reduction amount of a polymer decreased along with the increase of the salt content, and besides has the advantages of excellent shale inhibition performance.