As the X-ray spectrum of tracer in inertial confinement fusion implosion target is usually used to infer electron temperature, density, and the mixture of fuel and shell, it is necessary to study the relation between the characteristics of X-ray emission spectrum and the implosion process, which is helpful for inferring the implosion status. Under the condition of SGIII prototype, approximately 0.5% atomic percent of Ar atoms are doped in an indirectly driven implosion target, X-ray spectrum of Ar is numerically simulated. In this article, the influences of line re-absorption effect, tracer concentration, and profile of fuel plasma state on the emission spectrum are studied. The relation between the temporal evolution of the emission spectrum and the implosion process is also investigated. It is found that as the tracer concentration increases up to ～1%, the X-ray intensity is enhanced, but the influence of line re-absorption becomes severe. Temporal evolution shows that the peak time of Ar X-ray intensity is almost the same as that of neutron production (the former delays about 15 ps, which usually cannot be resolved). As is well known, the strong line emission occurs in the plasma with high temperature, high electron density, and proper ionization. The detailed analysis shows that at the peak emission time, as the core Ar plasma is over ionized, the main X-ray line emission region is located near the boundary region of the fuel, and this thin shell, whose thickness is about 4 μm and whose volume accounts for 56% of the total fuel plasma volume, emits the X-ray whose intensity is about 72% of the total line intensity. Therefore, the space-averaged plasma temperature and density, which are obtained by fitting the emission spectrum, mainly reflect the plasma state in this region.