The invention relates to a special sensitizer for explosive, in particular to a foaming agent for emulsion explosive, which comprises an A agent and a B agent, and the A agent and the B agent are mixed according to the weight ratio of 1: 1.2-1.5, wherein the A agent comprises the following components by weight percent: 0.5%-1.5% of thiourea, 3%-5% of potassium nitrite, 20%-30% of sodium nitrite and 63.5%-76.5% of water; and the B agent comprises the following components by weight percent: 7%-11% of ammonium nitrate, 0.2%-0.6% of phosphoric acid, 0.5%-2% of diammonium phosphate, 0.5%-1.5% of sodium thiocyanate, 1.8%-2.5% of citric acid, 0.7%-1.5% of glacial acetic acid, 0.7%-1.5% of sodium bicarbonate, 0.2%-0.9% of thiourea, 0.2%-1% of sodium nitrate, 0.9%-1.5% of urea and 76%-87.3% of water. The foaming agent has the characteristics of short foaming time, appropriate after-effect, easiness in control of density, good safety performance, long natural storage period and the like.