The invention relates to a preparation method of 
sodium alginate and belongs to the field of preparation methods for food additives. The preparation method comprises the following steps of cleaning, 
drying and sectioning fresh 
kelp, putting the fresh 
kelp into a conversion tank, adding 
formaldehyde with the weight being 10% of the weight of dried 
kelp for soaking, adding an alkaline-containing solution into the conversion tank, stirring for reaction, so as to obtain pasty glue solution, adding water to dilute the pasty glue solution, foaming the diluted glue solution by a foaming 
machine, flowing the diluted glue solution into a floating 
sedimentation tank, collecting 
clear liquid at the middle-lower part to enter a decalcifying tank, simultaneously adding a diluted 
hydrochloric acid solution so that flocculent 
calcium alginate becomes insoluble 
alginic acid, crushing 
alginic acid, adding a pure-alkaline solution into a neutralizing 
machine with the alkaline-adding amount being 10-20% of the weight of 
alginic acid, and stirring for reaction so as to obtain a 
sodium alginate solution, and finally 
drying the 
sodium alginate solution in a 
drying machine to obtain the 
sodium alginate. The preparation method has the characteristics of simple process, strong 
operability and the like, and is suitable for industrial production and high stability of products. The 
viscosity of the prepared 
sodium alginate reaches more than 1500cps, and is generally more than 1.5 times higher than the 
viscosity 1000cps in the traditional preparation method.