In this paper a model for calculation of heat transfer of the cathode assembly in vidicon has been given. Five cathode assemblies with different constructions have been calculated by computer. The calculated results prove the model is reasonable. In this paper, there are the following calculations: the temperatures at each part of the assembly; the heat radiation and conduction between different parts; the total power dissipation of the assembly and the influences of changing physical or geometrical parameters of the parts of the assembly on the totl power dissipation. Thi paper is helpfull to design a new cathode assembly.The design and experiment of the new cathode (Ba-Ni) assembly with small power dissipation is successful. The advantages of the new assembly is firm construction, easy assembling and quick heating (arriving at the operation temperature in a few seconds).