As a new type of circuit interrupter, DC thyristor breaker is very suit for high power DC system, such as subway, poloidal field power supply system of TOKAMAK (nuclear fusion experiment device), in which the transfer principle and force commutation technology was applied. In this paper, firstly, design of high power DC thyristor breaker has been finished, including main circuit design, snubber circuit design, commutation circuit optimize design, charger circuit design. Secondly, the reliability analysis of high power DC thyristor breaker has been presented, such as turn-off reliability analysis, turn-on reliability analysis, on-state and off-state reliability analysis. In these design and reliability analysis, some important conclusion have been derived, especially such as optimize design formulation of commutation circuit, the influences of distribution resistance in commutated circuit, the influence of stray inductance and their dispersivity in parallel connection thyristor branches for turn-off reliability, the influences of snubber circuit design for turn-on reliability, on- state and off-state reliability and so on. At last, we finished development and experiment of a bi-directional DC thyristor breaker for the poloidal field power supply system in EAST Super-conductive TOKAMAK Experiment result validated the correctness of the work above.