To meet the compact voltage-sharing configuration requirements of high-voltage multi-break tank vacuum circuit breakers, a simulation study on post-arc particles in a double-break series vacuum interrupter under main shield voltage-sharing was conducted. A PIC computational model for the post-arc phase in a double-break series vacuum interrupter under main shield voltage-sharing was established, revealing the variations in post-arc particles, post-arc potential, post-arc current, and post-arc sheath thickness. The study indicates that main shield voltage-sharing affects the development of the post-arc sheath in the double-break series main shield voltage-sharing vacuum interrupter. Compared to the traditional break voltage-sharing method, the main shield voltage-sharing method resulted in the smallest peak value of post-arc current (10 A) and the best consistency in sheath development between the series breaks (consistency coefficient approaching 0). This research provides valuable reference for the design of series main shield voltage-sharing vacuum interrupters in multi-break tank vacuum circuit breakers.