This invention relates to method and apparatus for preparing cyclic trimethylolpropane formal from heavy trimethylolpropane residual liquid. The method comprises: adding heavy trimethylolpropane residual liquid and strong-acidic catalyst methanesulfonic acid or toluene sulphonic acid into a reaction/rectification column bottom, reacting and rectifying. The weight of the catalyst is 0.5-2% that ofheavy trimethylolpropane residual liquid. The vacuum degree of the reaction/rectification column is (-0.095)-(-0.098) MPa, and the temperatures of the column bottom and overhead are 180-230 deg.C and135-140 deg.C, respectively. The content of cyclic trimethylolpropane formal in the fraction at the overhead is 97 wt. %, and the purify after secondary rectification is 99.6 wt. %. When the temperatureof the column bottom is heated to 140 deg.C within 2-4 h, alkali is added into the column for neutralization, and cyclic trimethylolpropane formal with a purity of 99 wt. % can be obtained by one-tim e rectification. The reaction/rectification column is a packed column or orifice column with 16 theoretical plates.