Disclosed are an albumin-free liquid formulation including a long-lasting oxyntomodulin conjugate in which an oxyntomodulin peptide comprising a derivative, variant, precursor or fragment of oxyntomodulin is linked to an immunoglobulin Fc region, which can increase the duration of physiological activity of the long-lasting oxyntomodulin conjugate and maintain the in vivo stability thereof for an extended period of time, as compared to native oxyntomodulin, and a method for preparing the liquid formulation. The liquid formulation contains a buffer, a sugar alcohol and a nonionic surfactant and does not contain a human serum albumin and factors that are potentially harmful to the human body, and thus is not susceptible to viral infection. The oxyntomodulin conjugate contains oxyntomodulin linked to an immunoglobulin Fc region, and thus has a large molecular weight, prolonged physiological activity, and excellent storage stability, compared to native oxyntomodulin.