The invention discloses a preparation method of an air entraining agent applied to preparation of a modified cassava 
starch-polycarboxylic acid composite water 
reducing agent, and belongs to the fieldof building materials. The preparation method of the air entraining agent comprises the following steps: putting 
rosin powder, p-toluenesulfonic acid and 
sodium hydroxide into a reaction kettle, adding water accounting for 50% of the total 
mass of the mixture A, mixing and stirring for 30 minutes, heating to 80 DEG C, keeping the temperature for 50 minutes, heating to 80 DEG C, and keeping the temperature for 45 minutes; heating the 
mixed solution A to 85 DEG C, slowly adding 
pentaerythritol and 
tall oil, keeping the temperature for 35 minutes, heating to 70-85 DEG C, slowly adding mahogany 
petroleum sulfonate, and keeping the temperature for 1 hour to obtain the air entraining agent. The air entraining agent has a good air entraining effect, the average 
pore diameter of air entraining bubbles is small, the durability of concrete is remarkably improved, meanwhile, the workability of the concrete is obviously improved, the 
slump loss of the concrete is reduced, and the strength of theconcrete is not reduced.