The standard "GB/T 20724-2021 Microbeam Analysis, Method for Determination of Film thickness by Convergent Beam Electron Diffraction of Thin Crystals" specifies a method for the determination of film thickness of thin crystal samples using Convergent Beam Electron Diffraction by Transmission Electron Microscope (EM) (TEM) or Scanning Transmission Electron Microscope (EM) (STEM). This method is suitable for thin crystals with linearity in the range of tens of nanometers to hundreds of microns and Film thickness in the range of tens of nanometers to hundreds of nanometers. The standard places particular emphasis on the determination of local Film thickness in the illuminated region of the electron beam due to the non-uniformity of Film thickness in thin specimens. This method provides reliable technical guidance for accurate measurement of Film thickness of thin crystal samples.