The invention discloses a first collecting mirror of an electron microscope, belonging to the field of electron microscopes. The first collecting mirror comprises a shell, an upper cover plate, a sealing space ring, a magnetic ring, a lead plate, a fixed diaphragm assembly, a pole shoe assembly, a magnetic circuit coil and a water jacket space ring assembly, wherein the shell is a cylindrical annular cavity with openings at two ends; the upper cover plate, the sealing space ring and the magnetic ring are connected and then arranged in the shell, the upper cover plate covers the opening at theupper end of the shell, the lead plate is arranged on the upper cover plate; middle parts of the upper cover plate, the sealing space ring and the magnetic ring are provided with mutually communicated through holes; the communicated through holes in the upper cover plate, the sealing space ring and the magnetic ring are internally sequentially provided with the fixed diaphragm assembly and the pole shot assembly, middle parts of the fixed diaphragm assembly and the pole shot assembly are provided with mutually communicated through holes which are used as electron beam channels of a first collecting mirror; and the magnetic path coil is arranged around the outside of the sealing space ring and the magnetic ring at the joint of the pole shoe assembly, the water jacket space ring assembly isarranged on the magnetic path coil, and the upper cover plate is arranged on the water jacket space ring assembly. The first collecting mirror has the advantages of simple structure, less parts, convenience for operation, assembly and maintenance, can be used for largely reducing electron beam spots on cross points of the electron gun, thereby improving properties of the electron microscope.