Isophthalaldehyde was prepared from m-xylylenediamine and hexamethylenetetramine under the control of the reaction pH. The optimum conditions for the synthesis were as follows: n(MXDA):n(HMT)=1:4.5, temperature 104℃, time 4h,pH 3～4, yield 76.2%. FTIR and 1HNMR and ESI-MS analysis identified the desired product. The product properties reached the international quality standards，and detected by HPLC, the purity of the target product was 99.7%.