A 3% aqua of hymexazol, mercaptophos and its preparation method are disclosed in the invention. The medicinal formula of the aque is as follows: 1.5-2.5 % 'Emeiling', 0.15-1.5% 'Jiashuangling', hydrochloric acid and water as remainder. The preparing steps are as follows. 0.096mol hydrochloric carboxyl-amine and 2N sodium hydroxide solution are added to 500ml flask, with PH value being controled at 8-12. When temperature of the solution is at -2 to 2 deg.c, 0.096mol acetacetic ether is dropped to the solution, with PH value bieng controlled at 8-12 by using 2N sodium hydroxide solution. After one hour of reaction, the above solution is added to 73ml strong hydrochloric acid, with the mixed solution being stirred for 8-14 hours as room temperature, so as to obtain the reactive solution. Adding mercaptophos to said reactive solution according medicinal formula gets the 3% aqua of hymexazol, mercaptophos.