The invention relates to preparation of escherichia coli competent cells and a transformation method of the escherichia coli competent cells. The preparation comprises following the steps of culturing the escherichia coli in a SOB liquid medium, adding a buffer solution I when OD 600 is 0.4 to 0.6, centrifugally collecting cells after ice bath, after a buffer solution II resuspends the cells, respectively putting into pre-cooling sterile centrifuge tubes to obtain the scherichia coli competent cells. In a transformation process, uniformly mixing DNA (deoxyribonucleic acid) with the competent cells, and directly coating the culture medium with the mixture after heat-shock, wherein ice bath and recovery steps are not needed. The escherichia coli competent cells are prepared by adopting buffer solutions of a special formula, the transformation efficiency can reach 1.23X109 cfu/mug pUC19 DNA. The method is simple in operation, high in transformation efficiency and good in repeatability, the preparation of the escherichia coli competent cells and DNA transforming operation are simplified, and a technological base is provided for high-efficiency and rapid transformation of the DNA.