The present paper introduces the method of retrieving the concentration of HCHO with differential optical absorption spectroscopy (DOAS).The authors measured ambient HCHO in Beijing region with the help of differential optical absorption spectroscopy instrument made by ourself,and discussed numerous factors in retrieving the concentration of HCHO with differential optical absorption spectroscopy (DOAS),especially,the choice of HCHO wave band,how to avoid absorption of ambient SO₂,NO₂ and O 3,and the influence of the Xenon lamp spectrum structure on the absorption of ambient HCHO.The authors achieved the HCHO concentration by simultaneously retrieving the concentrations of HCHO,SO₂,NO₂ and O 3 with non-linear least square fitting method,avoiding the effect of choosing narrow wave of HCHO and the residual of SO₂,NO₂,O 3 and the Xenon lamp spectrum structure in retrieving process to attain the concentration of HCHO,Finally the authors analyzed the origin of error in retrieving the concentration of HCHO with differential optical absorption spectroscopy (DOAS),and the total error is within 13.7% in this method.