A comprehensive analysis method is proposed to resolve the problem of simulating the complex thermoflow with two kinds of distinct characteristic lengths in a dry gas seal. A conjugated simulation of the complicated heat transfer and the gas film flow is carried out by using the commercial computational fluid dynamics (CFD) software CFX. By using the proposed method, three-dimensional velocity and pressure fields in the gas film flow and the temperature distribution within the sealing rings are investigated for three kinds of film thickness, respectively. A comparison of thermohydrodynamic characteristics of the dry gas seal is conducted between the sealed gas of air and helium. The latter one is used in a helium compressor for a high-temperature gas-cooled reactor (HTGR). From comparisons and discussions of a series of simulation results, it will be found that the comprehensive proposal is effective and simulation results are reasonable. Even under a hypothetical accidental condition, the maximum temperature rise in the dry gas seal is within the acceptable range of HTGR safety requirements.