The invention provides a method for constructing green alga phase in a saline-alkali soil underground water shrimp industrial culture pond, and belongs to the technical field of aquaculture. The green alga is platymonas helgolandica. The method comprises the following specific steps of: constructing an alga phase during a preparation period, maintaining the alga phase at the early stage of culture and maintaining the alga phase at the middle-late stage of culture. According to the method, euryhalic, high-temperature-resistant and high-adaptability platymonas helgolandica is adopted to construct the alga phase, so that the water quality of brackish water shrimp industrial culture is optimized, and healthy and rapid growth of shrimps is promoted.