X-ray spectrum of tracer in ICF implosion target is usually used to infer electron temperature, density and mix of fuel. As the plasma in fuel is in non-local thermodynamic equilibrium (non-LTE), a line transfer code Alpha is developed. Taking the electron temperature and density provided by radiation hydrodynamic as input condition, atomic kinetics and radiation transfer equation are self-consistently solved with the detailed configuration atom (DCA) model. The opacity for specified frequency intervals is obtained, and X-ray spectrum in the image plane is also presented.As application of Alpha program, the spectrum of doped Ar in implosion target on SG Ⅱ laser facility is simulated. The effect of self-absorption of K α line is studied. And it is shown that self-absorption of K α line affects both the intensity and shape of the spectrum, and it should be considered in simulating X-ray spectrum of Ar. And as the spectrum of local thermodynamic equilibrium (LTE) simulation gives large intensity and different shapes compared with the non-LTE results, non-LTE simulation is necessary in such a simulation.