The preparation of competent cell is the central step of bacteria transformation and has a great impact on transformation efficiency of nucleic acid. The aim of the research was to study the factors such as ionic species, strain types, plasmid concentration, ice-bath time and incubation time. The result shows that ionic species is fatal to the transformation efficiency, preparation of competent cells by monovalent ions (Li(+),Na(+),K(+)) and tervalent ion (Al(3+)) do not have capacity of transformation, preparation of competent cells by bivalent ion (Ba(2+),Ca(2+),Mn(2+),Mg(2+),Sr(2+)) has capacity of transformation. On the whole, the efficiency of Ca(2+) was found to be the optimum bivalent ion, following by Sr(2+),Mn(2+),Mg(2+),Ba(2+). On the other factors that affecting transformation efficiency, the transformation efficiency was the best when plasmid concentration was 100ng/mL, and ice-bath time should be controlled at about 30 min and incubate time selected as 60 min. Experiment of natural transformation later proves the existence of the phenomenon of natural transformation and it's also related to factors such as calcium chloride concentration, plasmid concentration and others.