The most challenging problem for simulating conjugate mass transfer problem is the variable concentration jump described by the double film theory at the phase interface. In the present paper, the physical mechanism of Henry's law is analyzed, and it is found that Henry's law can be expressed as a relation between two constant weight coefficients in lattice Boltzmann method (LBM). As a consequence, a unified multi-relaxation-time (MRT) lattice Boltzmann equation (LBE) for conjugate mass transfer is proposed. The asymptotic analysis proves that the interface conditions, including the concentration jump determined by the double film theory and mass flux continuity, can be ensured with second-order accuracy. Five test cases containing both transient and steady conjugate mass transfer problems with straight or curved interfaces are calculated to validate the present method. The results show that the proposed MRT LBE has both simplicity and good accuracy. It is capable of simulating conjugate mass transfer at both steady and transient periods, with a stationary or moving interface. Especially, it has a good numerical stability at large distribution coefficient and diffusivity coefficient ratio. (C) 2019 Elsevier Ltd. All rights reserved.