The invention relates to water-soluble fertilizer containing alginate and various nutrients and a preparation method of the water-soluble fertilizer. The water-soluble fertilizer comprises alginate, macro-elements N, P2O5 and K2O, medium elements Ca and Mg and trace elements Cu, Fe, Mn, Mo, Zn and B, wherein the weight percentage of the alginate is 3%., the total weight percentage of the macro-elements is 57%, the total weight percentage of the medium elements is 1%, and the total weight percentage of the trace elements is 0.64%. The preparation method includes: weighing the raw materials according to the weight percentage, and crushing and sieving; weighing the alginate, and evenly mixing with nitrogen fertilizer for standby; weighing the trace elements, and mixing with calcium and magnesium fertilizer for standby; weighing phosphorus fertilizer and potassium fertilizer, evenly mixing, adding the mixture, and sufficiently mixing to obtain the water-soluble fertilizer. By the water-soluble fertilizer, fruit yield can be increased by 20-30%, vegetable yield can be increased by 25-35%, and flower quality can be increased evidently.