Effects of material ratio, heating temperature, dosage of catalyst and reaction time on the conversation rate of methanol ,selectivity of methanol and purification of methylal were investigated in the process of preparing methylal with methanol , paraformaldehyde as materials and sulfuric acid as catalyst. The optimal preparation conditions were obtained by orthogonal experiment with results as follows: n(methanol):n(paraformaldehyde)=1.9, heating temperature 70℃, reaction time 2.75h, v(sulfuric acid):v(methanol) =3.5:100 and conversation rate 99.32%, selectivity of methanol 99.88%, mass fraction of methylal is 96.88%(content of methanol 3.11%, content of water less than 0.01% ).