The present invention relates to a method for preparing trimethylolpropane (TMP) comprising the steps of: 1) synthesizing trimethylolpropane by using n-butyl aldehyde, an aqueous solution of formaldehyde and an aqueous solution of alkali metal hydroxide through aldol condensation reaction and Cannizzaro reaction; 2) extracting trimethylolpropane from a resultant mixture of the step 1) by contacting the resultant mixture with an alcohol having 6 to 10 carbons; 3) removing alkali metal ion from a resultant extract of the step 2) by contacting the resultant extract with water; and 4) distilling the alkali metal ion-removed extract obtained from the step 3). According to the present invention, a separate formaldehyde recovery process can be omitted, the extraction efficiency of TMP can be maximized with using a relatively small amount of extraction solvent, the separation and recovery processes for extraction solvent can be simplified since a mixture of solvents is not used for TMP extraction, and the yield of TMP can be maximized while the amount of generated waste water can be minimized, thereby producing TMP economically with good efficiency.