dimethoxybenzaldehyde, as a valuable intermediate for drugs, is unconveniently synthsized in usual way. Reimer-Tiemann reaction and a methoxylation through dimethyl sulfate are employed for the preparation of 2,5-dimethoxybenzaldehyde from p-methoxyphenol in this paper. Comparing different phase transfer catalysts,it can be demonstrated that the polye thene glycel 10 000(PEG 10 000) is the most favorable catalyst used in Reimer Tiemann reaction. The yield and quality are improved through the methoxylation of 2 hydroxy 5 methoxy benzaldehyde in a buffer solution, The diverse factors influcencing the yield are examined. Some explanations are given from theoretical viewpoint. The proper experimental conditions are found and the overall-yield reached 68%.