The designed electron diffraction system consists of an ultra-fast electron gun,a sample chamber,a readout system,a power supply system,and a vacuum system,and it bears such unique characteristics as high energy,high temporal resolution,and high detection capacity.The photocathode is of a 35 nm Ag film deposited on an MgF₂ glass disk,and it is sensitive to ultraviolet light with wavelength of 266 nm. A magnetic lens is used to focus the electrons. Two pairs of electric deflection plates are used in the X and Y directions respectively to control the movement of the electrons,and one pair of them will act as a scanning plate while measuring the pulse width of electrons. The sample chamber is made of stainless steel,and in the middle of the chamber there is a specimen holder,capable of shifting in three dimensions and turning around its axis. The diffraction pattern recording system has a very high detecting efficiency,and even a single electron could be detected. A cascade MCP detector is used to ensure an electron gain reached to 10⁴. The electron gun is in a vacuum system of 10^{－4 Pa. The whole gun is shielded by a μ-metal sheath. The designed temporal resolution of the ultra-fast electron diffraction system(UED) is about 358fs.}