A sealed transmission device for a helium space of a high temperature gas cooled reactor and a drive device thereof are disclosed. The helium space (1) is enclosed by a chamber body (2), a pressure-bearing casing (3), a fastener (4) and a sealing element (5). A transmission member (8) is connected with an executing member (6) inside the helium space (1) and a motive member (7) outside the helium space (1) and transmits movements between them. The transmission member (8) is a magnetic shaft coupling which includes the pressure-bearing casing (3), a driving magnetic component (9) arranged outside the pressure-bearing casing (3) and a driven magnetic component (10) arranged inside the pressure-bearing casing (3). The motive member (7) and the driving magnetic component (9) are connected to each other to form a driving member, and the executing member (6) and the driven magnetic component (10) are connected to each other to form a driven member. The present invention can seal the thermal state radioactive helium space fully, and satisfy controllability and reliability requirements for the operations, such as big torque, long time and continuity, reciprocating rotation and frequent start and stop, etc.