A thermionic cathode electron gun of a pulse width of over 100 S and wide-beam section has been developed. Properties and structure considerations of this electron gun are given. Phenomena of vacuum electrical breakdown in electron gun has been experimentally studied, and main facfors resulting in vacuum electrical breakdown in electron gun found. Dependence of beam current transmitted through electron beam window on heating current of the cathode was measured. This electron gun can be widely used in the research of CO lasers, CO2 lasers and excimer lasers.