The cathead composite tower has been firstly used in domestic 500 kV circuit transmission line. Owing to its compact arrangement of post insulators and tension insulators at the end of cross-arm, the uneven electric field distribution is a prominent problem in practical operation that needs to be solved. A 500 kV cathead composite tower is chosen as the research object, and the 3D simulation model has been established with finite element software. The electric field concentration regions of middle phase and side phase are computed and then analyzed with finite element method (FEM). The potential distributions of composite cross-arm are compared and analyzed with normal 500 kV line insulators. The grading and shielding devices for this 500 kV cathead composite tower are designed. The influences of different structural parameters of each grading ring on the field strength at the key position of the composite cross-arm are studied with submodeling technology to save computing time. The results show that the key positions beard very high electric field intensities in initial scheme. It is not conductive to the long term stable operation of composite tower. The 500 kV composite cross-arm has better potential distribution than that of normal 500 kV line insulators. With the configuration of reasonable grading and shielding devices, the electric field distribution on key positions could meet with the critical value.