The invention provides a preparation method of a building concrete water reducing agent.The preparation method is characterized by comprising the steps that Chinese chestnut thorn shells are crushed with a plant crushing machine, Chinese chestnut thorn shell powder and 0.6 g of diatomaceous earth are taken and added into water to be heated to 40 DEG C, constant-temperature stirring is performed for 2 h, and obtained suspension is filtered to obtain leaching liquor; filtrate is concentrated to obtain concentrated liquor, 1.5 g of starch and lspan-80 are added into the concentrated liquor, stirring is performed for 0.5 h at 1000 rpm, and emulsion with the uniform shape is obtained; the emulsion is concentrated, and spray drying is performed to obtain modified tanning extract particles; an olefin compound, a catalyst, water, a catalytic synergist A and an emulsifier are added into 35 g of the obtained modified tanning extract particles, and reacting is performed for 1-3 h at the temperature of 70 DEG C-90 DEG C; sodium hydrogen sulfite and a synergist B are added, reacting is performed for 2 h at the temperature of 60 DEG C-80 DEG C, obtained emulsion is concentrated again, and spray drying is performed to obtain the building concrete water reducing agent.The preparation method is easy to operate and friendly to the environment, and the polysulfonate cement water reducing agent prepared through the method has the excellent water reducing performance and dispersing performance.