The diffraction field distribution of a high density grating with a period of 2.5λ is analyzed with the finitedifference time-domain (FDTD) method.The numerical results show the axial spatial evolution of the optical field near the 1/2 Talbot plane of the grating,which is verified by experiment with the scanning near-field optical microscopy (SNOM) technique.It should be helpful for understanding more clearly the diffraction behavior of a high density grating in micro- and nano-optics and be beneficial for applications of the Talbot effect,such as the near-field photolithography.