An image data decoding method of an image vertical blanking interval (VBI) and device thereof can adjust a run-in clock signal of data lines of teletext to a data phase of teletext data lines. The method can accurately decode data of the teletext data lines to avoid a phase bias and an erroneous decoding result. A main technical method to decode the data of the VBI is to extract the data of the teletext data lines to determine corresponding bit logical values of the image data and then to output a decode result and also output a phase adjustment value. The phase adjustment value is used to adjust a read phase value of the extracted image so as to synchronize a data phase in VBI.