The invention provides a preparation method of a compound as described in the formula 1. The method comprises the steps of (1) contacting a compound as described in the formula 2 and a compound as described in the formula 3 in the existence of alkali to obtain a compound as described in the formula 4; and (2) directly contacting a reaction product obtained in the step (1) and paraformaldehyde to obtain the compound as described in the formula 1, wherein weak acid is added in the reaction product before the reaction product obtained in the step (1) is in contact with the paraformaldehyde so that the residual alkali in the reaction product can be neutralized. By using the method, the compound as described in the formula 1 can be effectively prepared.