It is of practical importance to develop form stable composite phase change materials (FSPCMs) for high temperature thermal energy storage. Carbonates are promising candidates as the phase change material and steel slag is a promising economical skeleton material. However, the molten carbonates (Na2CO3 and K2CO3) react with steel slag (SS) at high temperature. Here, a two-step process was developed to overcome this problem. Firstly, the modified steel slag (MSS) was obtained by activation of the two carbonates, and secondly, FSPCMs were synthesized by mix-sintering method. The obtained K2CO3/KMSS FSPCMs kept better shape than that of Na2CO3/NMSS FSPCMs after the thermal cycles. Further test showed that K2CO3 had good chemical compatibility with KMSS and was uniformly distributed. With the increased K2CO3 content, the latent heat of K2CO3/KMSS FSPCMs gradually increases, and the measured latent heat was consistent with the calculated values. With mass ratio 4:6 (40K2CO3/60KMSS), the developed FSPCM showed the best thermal stability, the latent heat attenuation was 7.08%, and the mass loss was 6.56% after 200 thermal cycles.