The invention relates to a method for reinforcing a soft soil foundation at a deep mixing pile, which comprises the steps of land formation, construction layout, spray mixing and sinking, curing agent grout mixing, stirring and hoisting, repeated spray mixing and sinking, and shifting, etc.; and the main technological parameters are as follows: 0.55 to 0.75 of water-cement ratio, 0.38 to 0.75 m/min of sinking and empty stirring speed, 0.3 to 0.5 m/min of hoisting speed, 18 to 30L/min of guniting quantity and 0.5 to 0.7 MPa of guniting pressure. Compared with the existing cement mixing pile foundation reinforcing process, the method provided by the invention has the advantages of appropriate water-cement ratio, high pile quality, high strength, less reinforcing agent consumption, low cost and remarkable economic benefit, and is easy to evenly stir; and meanwhile, the method also has the following advantages: (1) the cement can be hydrated fully, and during construction, the role of the cement can be exerted completely; (2) the construction quality is easy to control and guarantee; and (3) the construction on deep piles can be performed.