A new double calcium treatment method was proposed according to industrial experiment. In the new process, the evolution of inclusion was analyzed and the controlling standard of T.[Ca]/T.[O] ratio for calcium treatment was discussed. The results show that good castability with low calcium content about 12 ppm was obtained by this method, and the inclusion grade of D type thin series decreases from 1.5 to 1.0 in final product. CaOMgOAl2O3 system inclusions containing small amount of CaO are mainly presented after LF refining. After the 1st calcium treatment, most inclusions are in the liquid region with low melting point and the MgO content decreases obviously. After the 2nd calcium treatment, the distribution of inclusion composition is more concentrated compared with the 1st calcium treatment, and the MgO content decreases further with the inclusion composition very close to CaOAl2O3 system and almost all in liquid region. In order to get a good calcium treatment result, T.[Ca]/T.[O] ratio should be controlled above 0.91, and the ratio between 0.91 and 1.25 is suitable in present work.