In response to the application requirements of high-resolution Reflection High-Energy Electron Diffraction （RHEED） in the fields of microelectronics manufacturing and surface analysis， and considering that a long working distance， micro-beam spot size， and small beam half-angle electron gun are key components for achieving high-resolution RHEED detection， a micro-beam spot RHEED quasi-parallel beam electron gun was developed. The characteristics of the electron gun's electron optical system were analyzed， and a low-aberration focusing magnetic lens was designed utilizing electron optical simulation software. An experimental platform was set up to measuring the beam spot diameter， beam current， and beam half-angle performance of the developed electron gun， as well as to conduct diffraction imaging tests on highly oriented pyrolytic graphite （HOPG） samples. The experimental results show that at a working distance of 500 mm， the beam spot diameter of the RHEED quasi-parallel beam electron gun is 47.1 μm （at an acceleration voltage of 30 kV）， The emission current and the beam half-angle are 144.96 μA and 0.289 mrad respectively （at an acceleration voltage of 15 kV）. Clear diffraction spots which intensity corresponding to the crystal structure factors were obtained on the HOPG sample.