In order to explore the optimal conditions for the synthesis of lignin ammonium salt embedding urea and its application in the slow-release fertilizer production, formaldehyde crosslinked lignin quaternary ammonium salt-urea (FCL-QA-U) was synthesized using reversed phase suspension polymerization technique with trimethyl lignin quaternary ammonium salt based on lignin (L-QA) as the raw material, formaldehyde as the crosslinking agent, and Span 80 as the dispersing agent. Its structure was analyzed by FT-IR. The effects of the dosage of crosslinking agent, rotating speed, reaction time,and the dosage of dispersing agent on the embedding rate of urea were studied for obtaining the optimal process conditions and product. Then slow-release performance was measured with the methods of water solving and soil column leaching. The results showed that urea was embedded in the L-QA, of which nitrogen content was 1.98%, and nitrogen content of FCL-QA-U was 3.27%. The optimal process conditions of FCL-QA-U were: the dosage of formaldehyde 2 mL, reaction time 2 h, rotating speed 300 r/min, and the dosage of dispersing agent 1 mL. Under the optimal conditions, the maximum embedding rate of urea by FCL-QA-U particles was 64.77%. The slow-release performance of FCL-QA-U particles accorded with the national standard of slow-release fertilizer GB/T 23348—2009, and it is a kind of slow-release fertilizer having both theoretical and also application value.