The invention relates to the field of composite fertilizers, and concretely relates to a long-acting modified sustained-release fertilizer coated by urea-formaldehyde resin. The fertilizer is prepared from the following raw materials in parts by weight: 10-15 parts of 1250-2000 mesh diatomite, 10-12 parts of ammonium phosphate, 12-15 parts of ammonium bicarbonate, 8-10 parts of calcium superphosphate, 6-8 parts of flaxseed meal powder, 4-6 parts of cobs, 8-10 parts of rapeseed meal, 4-5 parts of traditional Chinese medicine residue, 2-4 parts of bamboo shreds, 3-5 parts of agarose, 1-3 parts of tris(hydroxymethyl)aminomethane, 2-4 parts of disodium ethylene diamine tetraacetate, 12-15 parts of urea-formaldehyde resin emulsion, 1-2 parts of disproportionated potassium rosinate, 0.2-0.3 part of sodium dodecanesulphonate, 2-3 parts of polyamide, 1-2 parts of potassium iodate, 3-4 parts of tricalcium phosphate, 2-3 parts of zinc gluconate and 4-5 parts of an auxiliary agent. The composite fertilizer is abundant in raw material kinds and comprehensive and rich in nutrients, contains abundant organics, and is capable of improving soil environment. Dual cladding on nutrients is formed by utilizing diatomite and a sustained-release agent in the production technology, the fertility is durable and stable in release, and the quality and the output of crops are effectively improved.