The invention relates to a process for preparing amphoteric ion polymer fluid loss additive, which is characterized in that the process for preparation comprises the following steps: firstly, dissolving acrylamide and 2- acryloylamino-2-methylpropanesulfonic acid into water according to the mass ratio which is 10:1-3:1, getting mixed solution A, secondly, leading the mixed solution A to react 3-6hunder the temperature which is 35 DEG C to 70 DEF C, and pH value which is 3-10 and under the condition of adding with initiating agent, and getting mixed solution B, thirdly, cooling the mixed solution B with ice-water bath, adding NaClO which is -10 DEG C-5 DEG C and contains NaOH according to the substance ratio which is -CONH2:NaClO: NaOH=1:0.1:10-1:1:10, reacting 5-10h in ice-water bath, getting mixed solution C, fourthly, adding HCl solution, and getting viscous products, fifthly, stirring and washing viscous products in carbinol to get white sediment, and then taking out sediment, granulating, drying, disintegrating, and then getting products. The process for preparation has the advantages of high-temperature resistance, salinity tolerance and low production cost, and the conditionof the process is moderate and controllable.