We create mock X-ray observations of hot gas in galaxy clusters with a new extension of the L-Galaxies semi-analytic model of galaxy formation, which includes the radial distribution of hot gas in each halo. Based on the model outputs, we first build some mock light cones, then generate mock spectra with the SOXS package and derive the mock images in the light cones. Using the mock data, we simulate mock X-ray spectra for the ROSAT all-sky survey, and compare the mock spectra with the observational results. Then, we consider the design parameters of the HUBS mission and simulate the observation of the halo hot gas for HUBS as an important application of our mock work. We find: (1) our mock data match the observations by current X-ray telescopes. (2) The survey of hot baryons in resolved clusters by HUBS is effective below redshift 0.5, and the observations of the emission lines in point-like sources at z > 0.5 by HUBS help us understand the hot baryons in the early universe. (3) By taking advantage of the large simulation box and flexibility in semi-analytic models, our mock X-ray observations provide the opportunity to select targets and observation strategies for forthcoming X-ray facilities.