In the course of processing geophysical well logging data or developing well logging interpretation software, it is important to know the format of well logging data file. In understanding the format of well logging data to be processed, much work can be done, such as reading data, transforming data format, processing data and plotting. On the basis of analyzing XTF file format of ECLIPS 5700 well logging system, a class of reading XTF format data was built with the object-oriented method based on the memory mapping file technique. This technique realizes fast reading of XTF format file with mass data and also provides the technical reference for format transformation and data processing of other well logging data formats.