Liquid carboxylated nitrile butadiene rubber (LXNBR) shows promising application in various fields. Facing the challenges in its preparation via radical emulsion copolymerization, such as gelation, molecular weight and carboxyl group content, novel approach has been developed for the controlled preparation of LXNBR via the metathetic degradation of nitrile butadiene rubber (NBR) with acrylic acid (AA) as chain transfer agent (CTA). Based on the molecular weight and carboxyl group content of the resultant products, the reaction pathway was proposed. Such understanding is expected to prepare LXNBR with designed Mn and carboxyl group content for various applications, by facile adjusting the reaction condition, mainly the feeding mode of AA, NBR concentration and reaction time.