A system and method for in-situ amelioration of saline-alkali soil using microalgae. Sewage and microalgal seeds are pumped into an microalgae growth pond for growing microalgae, and online equipment is used for monitoring the level of nutrients in the pond. When the microalgae reach the stationary phase of their growth, the screen mesh is lifted to separate microalgae from liquid, so that they are conveniently harvested by scraping. The harvested microalgae are returned to the soil as a soil ameliorant.