Disclosed is a liquid formulation in which a long-acting INFα conjugate that has improved in vivo duration and stability can be stored stably for a long period of time. It comprises a stabilizer comprising a buffer, a sugar alcohol, a non-ionic surfactant and an isotonic agent. Being free of human serum albumin and other potential factors harmful to the body, the liquid formulation is free of concerns about viral infections and guarantees excellent storage stability to long-acting INFα conjugates.