The invention discloses a method for preparing nano-powder with a core-shell structure assisted by in-situ co-precipitation with surfactants. The preparation process is as follows: Weigh the nitrates of barium, strontium and magnesium according to the precise stoichiometric ratio and dissolve them in water to prepare A solution, and press Accurate stoichiometric ratio Weigh butyl titanate and dissolve in ethanol-oxalic acid solution to prepare B solution, weigh a certain amount of surfactant and dissolve in hot water-alcohol to prepare C solution, slowly drop ammonia into A, B and The co-precipitation reaction was carried out in the mixed solution of C, and the pH of the reaction system was controlled to be 2. After the precipitate was filtered, washed, heat-treated and ground, Ba1-nSrnTiO3@MgO core-shell nanopowder with high crystallinity was obtained. The preparation process of the invention is simple, the cycle is short, and the cost is low, and the obtained nano-composite powder has good dispersibility, uniform particle size, high crystallinity and coating structure, which can achieve the purpose of practical application.