The China Fusion Engineering Test Reactor (CFETR) is a new superconducting TOKAMAK fusion reactor. In Central Solenoid (CS) magnet of CFETR, the highest magnetic field in CS magnet will get 17.5 T, thus Bi-2212 round wires were employed for hybrid magnet design. Considerable lengths of conductor are needed to wind the CS magnet, hence, the CS coils comprise several conductor unit lengths connected by joints. In this paper, a design of sub-cable butt joint for Bi-2212 CICC connecting was introduced in detail, the joint assembly procedure was established to be 8 steps. The joint resistance was evaluated by COMSOL multiphysics in detail. THEA code was improved for Bi-2212 CICC with joint simulation, in the model, the joule heating was applied as the heating input, then the temperature rise from joule heating of joint was analyzed. The temperature margin at location of the joint was calculated in detail.