The invention discloses a preservation method of a liquid bacterial agent for sewage treatment. The preservation method comprises the following main steps: 1) regulating the pH value of the liquid bacterial agent to 6-6.5 by using a buffer solution; 2) removing dissolved oxygen from the liquid bacterial agent by using nitrogen; 3) adding a deoxidizer and a bacterium inhibitor; 4) covering with liquid paraffin, and sealing for preserving. By adopting the preservation method, 12 months later, the viable bacteria amount and the degrading activity can respectively reach 65% and 75% above.