With excellent response characteristics, high detection sensitivity and low operational cost, differential optical absorption spectroscopy(DOAS) is a powerful tool to trace concentration variation of trace HCHO.The accurate measurement of HCHO is achieved through improving the ratio of signal to noise and correcting the background scattering light, and the retrieving method of HCHO is developed including the selection of retrieval wavelength, the removing of interfering structures, steps of data processing and the spectrum dealing with nonlinear method. The HCHO concentration retrieved in different wavebands are compared. As a result the band from 314 nm to 332 nm is most suitable for the retrieval of HCHO.