The phase equilibrium and transition in the pseudo-binary system Ba2CaWO6-Sr2CaWO6 have been investigated by means of DTA, X-ray powder diffraction, precise measurement of lattice parameters and other methods. A continuous solid solution was formed at high temperatures (above 860℃). At room temperature, a solid solution based on the structure of Ba2CaWO6 was formed on Ba2CaWO6-rich side and a solid solution with the structure of Sr2CaWO6 on Sr2CaWO6-rich side and a phase transition point was found at 25mol. per-cent of Ba2CaWO6.As a coating material for the filaments of high voltage sodium lamp, the important characters of BaxSr2-xCaWO6, the electron emission and the rise of the lamp voltage during multiswiching, were investigated. The best performance was found at the composition of BaSrCaWO6.