The invention relates to a preparation method and application of a carbonyl cobalt catalyst through which 3-methyl lactate is prepared in an ethylene oxide methyl esterification mode. The preparation method of the carbonyl cobalt catalyst is characterized in that cobalt powder is used as a reducing agent, soluble cobalt salt and cobalt powder and an auxiliary reducing agent are placed into a high-pressure reaction kettle according to a mole ratio to be sealed, gas in the reaction kettle is replaced with inert gas, and stirring, cooling and centrifugal separation are carried out to obtain the carbonyl cobalt catalyst. The preparation method of the carbonyl cobalt catalyst is low in cost and has the better activity and selectivity compared with other preparation methods of the carbonyl cobalt catalyst, and no other carbonyl compound is produced in the preparation method; however, if manganese powder, zinc powder, iron powder, nickel powder and the like are adopted, corresponding carbonyl compound by-products are produced, impurities are introduced, and then the catalytic effect of the catalyst is affected. The selectivity and yield of the carbonyl cobalt catalyst through which the 3-methyl lactate is prepared in the ethylene oxide methyl esterification mode are high.