The invention relates to a long acting compound sulfamonomethoxine suspension which consists of 10g of sulfamonomethoxine, 2g of trimethoprim, 0.3g of sodium hydroxide, 0.3g of carboxymethylcellulose sodium, 30g of propylene glycol and 0.05g of tween-80. A preparation method thereof comprises the steps that: (1) the carboxymethylcellulose sodium swells by using 20ml of water used for injection; (2) then the propylene glycol is added; (3) the sodium hydroxide is dissolved with 5ml of water for standby; (4) the products of Step (1) and Step (2) are placed upside down in a beaker, added with the trimethoprim, the sulfamonomethoxine and the tween-80 in sequence and stirred while being adding; the sodium hydroxide is used for adjusting and controlling the pH value to be 5.3 to 5.6; and (5) the solution is put into a colloid mill so as to be milled for 10 minutes, thus obtaining the compound sulfamonomethoxine suspension. The veterinary drug has the advantages of long half life, less untoward effects, good curative effect, and the like.