The invention relates to a preparation method of synergistic slow-release coated 
fertilizer. The preparation method comprises the following steps of S1, preparation of a first 
coating material: agitating 
urea and a 20% to 40% of 
formaldehyde solution in a reaction tank until the 
urea is completely dissolved, regulating the pH of a solution to 8 to 9 by using 
triethanolamine, reacting for 1h to 2hat a normal temperature, regulating the pH to be between 5 to 6.5, heating to 60 DEG C to 80 DEG C while agitating, continuously agitating, and stopping heating until a 
reaction system approaches a 
gel state, so as to obtain the first 
coating material; S2, preparation of a second 
coating material: uniformly mixing 70% to 99% of 
humic acid, 0.5% to 1% of activator and 0.5% to 29% of polynucleic acid 
powder according to percentage by weight, so as to obtain the second coating material; S3, raising the temperature of a 
urea master batch with a particle size range of 2.0mm to 5.0mm to 60 DEG C to80 DEG C in a revolving drum, spraying the first coating material to the surface of the urea master batch through a sprinkling device, and 
aeration-
drying, so as to complete a first layer of coating;and S4, preheating 
fertilizer completed in the first layer of coating, preheating to 40 DEG C to 50 DEG C, spray-coating an 
adhesive (dissolving 
alginic acid in an alkaline solution) to the surface offluidized 
fertilizer, and then dispersing the second coating material on the surface of the fertilizer, so as to obtain the synergistic slow-release coated fertilizer.