The invention provides a low explosive for metal welding. The low explosive comprises, in percent by weight, 27-30% of urea, 27-30% of nitric acid, 18-20% of ammonium nitrate, 0.10-0.15% of octadecyl primary amine, 21.90-25.85% of deposit white carbon black, and water. The low explosive is prepared according to the following preparation method: firstly adding 27-30 Kg of urea into 30-35 Kg of water according to the batches, heating to 30-40 DEG C, stirring, after dissolving, adding 27-30 Kg of nitric acid at a uniform speed within 10 min, and stirring for 30 min, so as to generate urea nitrate; adding 18-20 Kg of ammonium nitrate and 0.10-0.15 Kg of octadecyl primary amine into urea nitrate, heating with stirring, controlling the heating temperature for 100-110 DEG C, after 30 min, performing vacuum drying and crushing; and adding 21.90-25.85 Kg of deposit white carbon black, and performing grinding and uniform mixing, so as to obtain the low explosive for metal welding. The material cost is low, production is simple, the detonation sensitivity is good, the detonation velocity is stable and adjustable, the explosive detonation velocity is 2000-2400 m/s, the storage life is within 1 year, the explosive does not cake, production is safe and simple, the employed materials all do not contain toxic and harmful compositions, and the production and usage processes are harmless to contact personnel and environment.