Abstract: Compared with ground VLBI, space VLBI provides higher angular resolution since its baseline is much longer. Our country has started the development of our own space VLBI science and technology. Russian space-VLBI project RadioAstron was finished in 2018. We can learn a lot from its successful experience. This project, which is quite different from ground-VLBI, uses a dedicated data format Radio Astron Data Format (RDF). Besides, the frequency reference is provided by the on-board or ground-based uplinking hydrogen atomic frequency standard. The data is quantized and transferred to the ground station, and is then labeled with time-tag and recorded in the hard disk. In this paper, we analyze the RDF format, carry out RDF data decoding and VLBI correlation. Moreover, we compare RDF with other commonly used VLBI data format, e.g., Mark5B and VDIF. Our work will pave the way for the format designing and data processing of space-VLBI data in China.