An apparatus for processing an audio signal and method thereof are disclosed. The method comprises receiving a downmix signal, object information indicating attribute of the object and including object number information, preset information to render the downmix signal, external preset information being inputted from external, and applied object number information indicating the number of object being applied the external preset information; determining whether the applied object number information is identical to the object number information; and rendering the downmix signal by using the external preset information, if the applied object number information is identical to the object number information, wherein the external preset rendering parameter renders the object being included in the downmix signal and the external preset metadata indicates attribute of the external preset rendering parameter. Accordingly, an audio signal can be efficiently reconstructed by individually selecting and applying external preset information by a data region unit or by selecting and applying the same external preset information to a whole downmix signal.