The invention discloses a method for quickly acclimatizing active sludge mixed flora to prepare polyhydroxyalkanoate (PHA) by using an aerobic dynamic drainage method and relates to a method for preparing the PHA. The invention solves the problems that the acclimatization time for synthesizing PHA through active sludge mixed flora is long, a reactor works unstably and the synthesized PHA is low in content in the prior art. The method comprises the following steps: 1, active sludge is put into a sequencing batch reactor (SBR); 2, 6-12 hours are taken as a cycle of operation, in the cycle of operation, two-time water introduction, precipitation and drainage are carried out, the drainage rate is 1:1, during the first water introduction, a carbon source and various nutrients are introduced, and during the second water introduction, only the supplement of nitrogen nutrient is carried out; 3, 2-4 cycles are operated every day, the sludge detention time is 1-10 days and sludge-water mixed liquid is collected; 4, batch experiments are carried out, half supernatant is discharged, equivalent carbon source is supplemented, aeration, precipitation and drainage are carried out, and the process is repeated for 6 to 8 times. The invention belongs to the field of biodegradable plastic synthesis.