The invention discloses a method for preparing a partially crosslinked and partially branched polymer oil displacement agent by controlling a dynamic condition. The method comprises main steps as follows: raw materials including acrylamide, diethylene glycol, polyethylene glycol, 2-(dimethylamino)ethyl methacrylate, potassium persulfate and sodium bisulfite for a formula are dissolved in deionized water accounting for no less than one time in parts by mass of the formula ratio of each material respectively to prepare solutions; acrylamide, 2-(dimethylamino)ethyl methacrylate, a diethylene glycol solution and polyethylene glycol are added to a reactor filled with water at the temperature of 0-60 DEG C for water bath, nitrogen is introduced with stirring to sufficiently remove oxygen in the reactor and the solutions, a potassium persulfate solution and a sodium bisulfite solution are added, after a reaction system starts polymerization, the viscosity is improved obviously, nitrogen introduction and stirring are stopped, and the colloidal oil displacement agent is prepared after the polymerization is performed to a constant temperature. Polymer oil displacement agents adopting different structures and having different properties can be prepared with the method, and requirements of different oil reservoir environments can be met.