The invention discloses pollution-free low-damage alkaline completion fluid capable of replacing CaCl2, and a preparation method of the pollution-free low-damage alkaline completion fluid. The alkaline completion fluid comprises the following components in percentage by mass: 30 to 70 percent of potassium carbonate, 1 to 3 percent of hydrophobic modified sodium alginate, 1 to 2 percent of an inhibitor, 1 to 2 percent of a filtrate reducer, 0 to 1 percent of a defoaming agent and the balance of water. The novel alkaline completion fluid capable of replacing CaCl2 saline completion fluid with the density being 1.40 g/cm<3> is prepared by taking hydrophobic modified sodium alginate as a completion fluid tackifier and functional component with environmental friendliness and high compatibility for the first time. The alkaline completion fluid is prepared by adding potassium carbonate, the hydrophobic modified sodium alginate, the inhibitor, the filtrate reducer and the defoaming agent into water sectionally and stirring uniformly. The preparation method is simple and safe, and has extremely good industrial prospect.