Disclosed is a method of preparing methylol alkanal. According to the present invention, a yield of methylol alkanal may be improved without using formaldehyde in an excess amount of a theoretical minimum molar ratio or more, formaldehyde wastewater may be reduced, and the amount of formaldehyde remainder, which may act as a hydrogenation catalyst poison, comprised in the methylol alkanal is decreased when the prepared methylol alkanal is hydrogenated resulting in improvement in efficiency of the hydrogenation.