The invention provides an oligomer surfactant, and a preparation method and application thereof. The oligomer surfactant has high surface activity, and excellent foaming and foam stabilizing properties. The oligomer surfactant has a specific structure: the molecule contains 3-7 hydrophobic chain segments and hydrophilic units, wherein each hydrophobic chain segment is a C8-C18 alkyl chain, and each hydrophilic unit is composed of 0-3 epoxy units and sulfate groups which are connected in the position near the hydrophilic unit through chemical bond covalence. The preparation method comprises the following steps: carrying out reaction on an alkyl epoxy compound B and a polylol A to generate an intermediate C, and sequentially modifying the hydrophilic epoxy units and sulfate groups on the intermediate C, thereby obtaining the oligomer surfactant. The oligomer surfactant has excellent air-entraining and foam stabilizing property when being used as a concrete air entraining agent; and meanwhile, the hardened concrete has excellent aerated structure and can not generate great impact on the setting strength of the concrete.