The property of the near field subwavelength imaging in a metal thin-film structure is investigated using finite difference time domain method based on Drude model. Using the transfer matrix method，the transmission of this structure for the evanescent wave component is given，which illustrates the influence on the subwavelength imaging quality. In practice a Cr grating mask with 1μm period is imaged onto a photosensitive material through a silver thin film structure by utilizing the optical near-field lithography techniques.