The uniform design method was used to screen the solidifying point depressing effects of 18 traditional diesel cold flow improvers on biodiesel derived from waste cooking oil. The cold flow improvers with good effects were selected for orthogonal optimization. Finally, the mixed cold flow improver (CFI) with the best depressing effect was selected to explore its depressing mechanism for biodiesel. The results show that the typical CFIs such as A132, A146, 10-320, 10-330, A-4, CS-1, AH-BSFH, Haote, T1804D, and HL21 all have a certain solidifying point depressing effect on biodiesel, while other cold flow improvers had no obvious effect. Amongst them, 10-330 (PMA polymer) and AH-BSFH (EVA polymer) had better solidifying point depressing effects over others, both of which reduced the solidifying point (SP) of biodiesel by 4 °C and the cold filter plugging point (CFPP) by 2 °C and 3 °C, respectively. From the orthogonal mixing experiment, it can be seen that the combination of 10-330 and AH-BSFH at a mass ratio of 1:8 had the best depressing effect, reducing the solidifying point and cold filter plugging point of biodiesel by 5 °C and 3 °C, respectively. Orthogonal analysis showed that when used in combination, AH-BSFH had a greater impact on the solidifying point, while the ratio of the combination had a greater impact on the cold filter plugging point.