The invention provides a passive spiral micro-structural mixing unit and application of the same in synthesis of a nanometer barium sulfate particle. The mixing unit is composed of a sample introduction pipe, a liquid storage chamber, a mixing channel, a buffer chamber and a sampling pipe, wherein the mixing channel, a core part for fluid mixing, is processed into a spiral structure with a hydrophobic material, and deflection baffles are uniformly arranged on the surface of an interior wall. The deflection baffles comprises front-tilting ones, vertical ones and back-tilting ones. The design of the deflection baffles is beneficial for generation of local secondary flow in the mixing unit, which strengthens fluid mixing. The mixing unit provided by the invention is applicable to a fast-reaction process which costs far less time in reaction than in mixing or a reaction-deposition process and has wide application prospects in considerable mixed reaction fields like medicine and chemical engineering.