The invention relates to a preparation method of urea formaldehyde liquid fertilizer. The preparation method comprises the following specific steps: a) mixing urea with formaldehyde to react to generate hydroxymethylurea; b) adding a proper of quantity of ammonia to react with the hydroxymethylurea to generate a triazinone compound, and obtaining the urea formaldehyde liquid fertilizer or carrying out next-step reaction after reaction finishes; c) adding a proper quantity of formaldehyde to continuously react, and obtaining the urea formaldehyde liquid fertilizer or carrying out next-step reaction after reaction finishes; d) adding a proper quantity of urea to continuously react, and obtaining the urea formaldehyde liquid fertilizer after reaction finishes. Compared with the prior art, the preparation method disclosed by the invention has the following beneficial effects: 1) a multiple-step method is adopted to prepare the urea formaldehyde liquid fertilizer so as to be more convenient to control a reaction direction and the content of various components in a product; 2) according to the growing periods and the fertilizer requirements of different crops, the liquid fertilizer with different component content, namely the special urea formaldehyde liquid fertilizer of different slow release periods, is prepared.