The invention discloses an alkali liquid cleaning agent for an outer railway wagon of a bullet train, as well as the preparing method and the application of the alkali liquid cleaning agent. The alkali liquid cleaning agent comprises organic base, organic acid, inorganic acid, organic solvent, corrosion preventive compound, nonionic surfactant, anionic surfactant and water. The preparing method comprises the following steps: heating water in an agitating pan to 40 to 60 DEG C; adding the organic base, the organic acid, the inorganic acid, the corrosion preventive compound, the nonionic surfactant and the anionic surfactant into the water in the agitating pan sequentially while stirring; stopping heating when all the raw materials are melted; cooling to about 45 DEG C; adding the organic solvent; and stirring for 20 to 100 min, so as to obtain the alkali liquid cleaning agent after the mixture is stirred uniformly. The alkali liquid cleaning agent can be used only by diluting the alkali liquid cleaning agent with water which is 5 to 20 times of the alkali liquid cleaning agent in size.