Xenon (Xe) is a noble gas with 54 electrons and belongs to the category of “Heavy” atoms in protein crystallography, where it has been used for phase determination of "unknown" protein . We successfully added xenon gas into the crystals of recombinant trichosanthin (TCS) protein under the pressure of 1 to 3 MPa. Using the single xenon derivative, the crystal structure of TCS was solved by single-wavelength anomalous diffraction (SAD) method without any known TCS structure information. It demonstrates an interpretable electron density map of xenon atom inside a cavity of TCS, which is formed by a set of residues with hydrophobic side-chain. Compared with the native TCS crystal structure solved in parallel, it is revealed that there is not significant backbone or side-chain conformation changes upon the binding of Xe atom. Moreover, in the first time, we identified one pre-existing hydrophobic cavity adopted for Xe binding. This Xe-binding cavity sits just under the surface of substrate binding site of TCS. Two hydrophobic residues (Ile71 and Phe83) act as a set of “swing-door” to connect the protein surface to the Xe-binding cavity. This structure property of TCS will make Xenon gas to diffuse easily into its insides under the moderate pressure, and to be hold steadily for a period to finish the diffraction data collection.Combined with the mica surface induced epitaxial crystal growth properties of TCS (Sun et al., 2010), we find that TCS would be a general applicable fusion tag in protein crystallography to solve the “unknown” protein structure especially in cases where selenomethionine incorporation may be difficult or impossible (i.e., in expression systems other than E. coli).