The invention provides a preparing method for heat resistance and salt tolerance polyacrylamide used in an oil field. The preparing method comprises the following steps that acrylamide, 2- acrylamide-2-methyl sodium, sodium acrylate, aiding solvent and deionized water are mixed to be dissolved fully and uniformly, and alkali is used for adjusting a pH value to obtain a homopolymerisation solution; the homopolymerisation solution is pumped into a polymerizing pot after being cooled, 2,2'-azobis-(2-methylbutyronitrile) is added, nitrogen is introduced, chain extender is added at first, then solutions prepared by complexing agents, oxidizing agents, reducing agents and auxiliary reducing agents are added sequentially, after all the solutions are added, nitrogen continues to be introduced, the polymerization reaction is carried out, heat is preserved, and the aging reaction is carried out to obtain a polyacrylamide rubber block; the rubber block is taken out, cutting, pelleting, drying, smashing and screening are carried out on the rubber block to obtain high-hydrolysis-degree polyacrylamide. The polyacrylamide obtained through the method has the advantages of stable quality, high viscosity, heat resistance, salt tolerance, high dissolving speed and good solubility, the product energy consumption is low, and in the high-temperature and high-salinity environment, the viscosity is degraded slowly.