Disclosed is a pure alkali metal seed injection system of an inert gas plasma magnetohydrodynamic generator. An inert gas carrying, conveying and control system, a Laval spray tube premixer (8) and a straight-pipe-shaped nozzle (9) of the pure alkali metal seed injection system are arranged sequentially from left to right. An alkali metal seed constant temperature storing and measuring pushing system is arranged in front of the Laval spray tube premixer (8). The Laval spray tube premixer (8) is communicated with the straight-pipe-shaped nozzle (9). The straight-pipe-shaped nozzle (9) extends into a mixing chamber (11) and is perpendicular to an inert gas working medium primary gas flow (10) pumped into the mixing chamber. Carried gas conveyed through the inert gas carrying, conveying and control system and seeds conveyed through the alkali metal seed constant temperature storing and measuring pushing system are mixed in the Laval spray tube premixer (8), and mixed gas carrying the seeds is formed. The mixed gas is fed into the mixing chamber (11) through the straight-pipe-shaped nozzle (9) and mixed with the injected inert gas working medium primary gas flow (10) in an obliquely reversed mode.