The invention relates to the technical field of weak alkali ternary injection distribution processes, in particular to a single-pump single-well weak alkali ternary injection device with two-stage concentration regulation and an application method thereof. The single-pump single-well weak alkali ternary injection device with two-stage concentration regulation comprises an AS (alkali-surfactant) storage tank, an S storage tank, an SP (surfactant-polymer) storage tank and a PLC (programmable logic controller); the AS storage tank is provided with an alkali mother liquid inlet, a first surfactant mother liquid inlet and a first water inlet, and the S storage tank is provided with a second surfactant mother liquid inlet and a second water inlet. The single-pump single-well weak alkali ternary injection device with two-stage concentration regulation and the application method thereof have the advantages that viscosity loss and concentration error of a surface target liquid can be decreased, the concentration of a polymer can be regulated flexibly by injecting surfactant solution, an alkali and a surfactant are distributed at a low-pressure end so that the prepared AS mixed liquid has uniform and stable alkali concentration, and the alkali concentration instability problem due to drip mixing of the alkali and surfactant at the high-pressure water end in the prior art is solved.