The invention discloses a nano-particle type polycarboxylic acid water reducer and a method for preparing the same. The nano-particle type polycarboxylic acid water reducer is prepared from unsaturated carboxylic acid compounds, unsaturated polyoxyethylene ether, sodium methallyl sulfonate, chain transfer agents, butyl acrylate, initiators and water by the aid of soap-free emulsion polymerizationprocesses. The pH (potential of hydrogen) value of the nano-particle type polycarboxylic acid water reducer is 6-8. The nano-particle type polycarboxylic acid water reducer and the method have the advantages that high-dispersion and high-water-reduction effects can be realized by the nano-particle type polycarboxylic acid water reducer for cement concrete, processes for the nano-particle type polycarboxylic acid water reducer are simple, industrialization can be facilitated, and environments can be protected.