NOVELTY - Changing composition of Escherichia coli fatty acid comprises: (a) recombining the gene encoding beta-hydroxydecanoyl acyl-carrier protein (ACP) dehydrase (FabA) and beta-ketoacyl-ACP synthetase I (FabB) to the downstream of the expression plasma starter and transforming the E. coli cells to obtain engineered E. coli strain; (b) inoculating the engineered strain; (c) suspending; (d) centrifuging; and (e) adding methyl ester reagent to the saponified system and extracting the fatty acid methyl ester (FAME).    USE - The method is used for changing composition of E. coli fatty acid (claimed).    ADVANTAGE - The method greatly increases the unsaturated fatty acid of engineered E. coli. The fatty acid composition from the engineered E. coli has low temperature liquidity.    DETAILED DESCRIPTION - Changing composition of E. coli fatty acid comprises: (a) recombining the gene encoding beta-hydroxydecanoyl acyl-carrier protein (ACP) dehydrase (FabA) and beta-ketoacyl-ACP synthetase I (FabB) to the downstream of the expression plasma starter and transforming the E. coli cells to obtain engineered E. coli strain; (b) inoculating the engineered E. coli strain to 400-500 ml of LB liquid culture medium containing 50 mu g/ml kanamycin; culturing by shaking at 35-37 degrees C at 200-250 revolutions/minute (rpm) until the optical density (OD)600 is 0.6-0.8; adding isopropyl-beta-D-thiogalactopyranoside (IPTG) inductive agent until the final concentration is 0.1-1 mmol/L; and continuing to culture for 3-24 hours and centrifuging and collecting the strain; (c) suspending the strain in 1-2 ml of deionized water and adding 5-10 ml of chloroform/methanol (volume ratio of 2:1); shaking for 3-5 minutes; and removing fats of cells; (d) centrifuging the extracting liquid in (c) at 8000-10000 rpm for 5-10 minutes; collecting the organic liquid in lower layer; drying by nitrogen; adding 2 ml of saponifier and reacting at 60-70 degrees C for 1-2 hours; and (e) adding 2 ml of methyl ester reagent to the saponified system; reacting in water bath at 60-70 degrees C for 0.5-1 hours; extracting the fatty acid methyl ester (FAME) by 2 ml of hexane; decompressing, distilling, and condensing to prepare biological oil.