The present invention discloses a fully soluble aquatic fertilizer and its preparation method and pertains to aquaculture field. The aquatic fertilizer of the present invention comprises monoammonium phosphate 15-45%, sodium humate 12-20%, sodium silicate 5-12%, compound premix 8-13%, and urea 30-40%. The compound premix is mainly composed of trace elements. Firstly, the aquatic fertilizer of the present invention has comprehensive nutrition and balanced formula; secondly, the present invention has high dissolution speed and utilization rate; thirdly, the present invention features simple use method, small use amount, sufficient content, long lasting time and low cost; fourthly, the present invention can not only promote the mass propagation of food organisms of aquatic animals but also specifically cultivate helpful algae, improve water quality, increase dissolved oxygen, effectively avoid pond poisoning, reduce diseases, lower feed coefficient and enhance fish immunity; the present invention is green and environment-friendly, and has stable quality and no harm on human and animal and no pollution on environment.