The invention belongs to the technical field of fertilizers and preparation of fertilizers, and discloses a root nutrient solution for cultivation of watermelons by a coco coir substrate in differentgrowth periods and a preparation method of the root nutrient solution. The root nutrient solution for cultivation of the watermelons by the coco coir substrate comprises the following components: calcium nitrate, 13% chelated iron, calcium superphosphate, potassium dihydrogen phosphate, magnesium sulfate, potassium nitrate, manganese sulfate, boric acid, zinc sulfate, copper sulphate, ammonium molybdate, potassium sulfate, sodium chloride, selenium sulphate, sodium metasilicate and auximone. In the earlier stage of growth of small watermelons, a nutrient solution formula in the first stage isused, and is mainly used for promoting nutrition growth, root systems of plants are developed, the plants grow robustly, and disease resistance is high; and in a blooming and fruit bearing stage, a nutrient solution formula in the second stage is used, the content of microelements is increased, absorption of fruits to the microelements is promoted, after the nutrient solution formula is used, human immunity can be enhanced, moreover, small watermelons of female flowers can be large, abortion is not caused easily, and the fruit expanding speed is increased obviously.