The invention discloses a preparation method of polyacrylamide for an oil field. The preparation method is characterized by comprising the following steps: taking an acrylamide water solution with the mass concentration of 23-30%, adding a cosolvent, stirring and uniformly mixing, adding an acid to regulate the PH value to 6-7, cooling to 2-5 DEG C, introducing nitrogen to remove oxygen in a system, sequentially adding an azo initiator, a complexing agent, a high-efficient chain transfer agent, a reducing agent and an oxidant when the oxygen content in the system is less than 0.001%, closing and sealing after the reaction of the system, performing heat preservation for 1-2h in a water bath at the temperature of 80 DEG C, taking out a colloid, mincing, then adding an inorganic base, performing hydrolysis for 1-2h at the temperature of 70-90 DEG C, and finally drying and crushing the colloid to prepare the high-score and high-viscosity polyacrylamide for the oil field. The polymerization operation is convenient, the reaction time is short and the production period is shortened; and the product has great solubility and filterability and is in full compliance with the standard of tertiary oil extraction of the oil field.