The invention provides a modified small-pore molecular sieve adsorbent, and a preparation method and a use thereof. The modified molecular sieve adsorbent comprises: a carrier; and an active componentloaded on the carrier, and the active component comprises an alkaline earth metal and/or a transition metal, wherein a weight ratio of the active component to the carrier is 0.01-0.5, preferably 0.01-0.3 and more preferably 0.02-0.2; and the carrier comprises a molecular sieve having an average pore size of 0.001-2 nm. The modified molecular sieve adsorbent can adsorb ammonia gas at normal temperature (20-30 DEG C), has the characteristics of large adsorption capacity and long penetration time, and can well meet the practical application requirements. The modified molecular sieve adsorbent ofthe present invention can be used for recovering ammonia gas generated by agricultural sources such as breeding industry plants, and also can be used to treat industrial flue gas and escape ammonia generated in motor vehicle (diesel vehicle) tail gas SCR systems.