TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: Preparing biodiesel using alga oil comprises: (a) preparing ethanol in the alga: (i) cloning gene: cloning PDC gene and ADH gene from cell; cloning FAEES gene from liver cell; (ii) constructing plasmid: linking PDC and ADH into plasmid pUC19; obtaining PDC+pUC19 and ADH+pUC19; preparing Escherichia coli cell; and transforming into the cell; (iii) expanding plasmid: expanding plasmid in the cell and extracting plasmid; (iv) constructing expression vector: cutting plasmid pUC19 with enzyme; carrying out electrophoresis; recycling aim gene by gel electrophoresis; cutting carrier with enzyme; obtaining adhesive tail end; linking the PDC gene and the ADH gene to the plasmid carrier; obtaining PDC+ carrier, ADH+ carrier, and FAEES+ carrier; (v) transforming into Agrobacterium rhizogenes: preparing A. rhizogenes competent cell; transforming the A. rhizogenes with the two plasmids obtained in the prior step; (vi) transfecting alga: culturing transformed A. rhizogenes and alga; inserting PDC gene, ADH gene, and FAEE gene into alga gene unit; (vii) genetic expression: culturing transgenic alga in large amount after washing the A. rhizogenes; and (viii) detecting ethanol: detecting change of content of ethanol in the substrate; if the ethanol is obviously increased, the two genes being successfully shifted into the alga; and (b) preparing biodiesel through ester exchanging with alga biological oil: cloning FAEES gene by liver cell; taking A. rhizogenes as medium; shifting the FAEES gene into the transgenic alga; detecting content of fatty acid; when the content does not change; the crushed alga cell collects cultured mixture through separating; producing biodiesel with catalyzing technique; the side product being glycol; producing biodiesel in the alga transforming plant and producing biodiesel out of the alga transforming plant. The method uses FAEES and fatty acid methyl ester synthetase (FAMES) enzymes in and out of the biodiesel. The alga comprises chlorella, microalgae, common alga and uncommon alga, or freshwater alga and salt water alga.