Abstract: The design of tumbler (precessional) centrifuge is based on the theory of precessional separation. It is a type of automatically,continuouslly operating filter-centrifuge. This article contains two aspects: I . According to the structure feature of the horizontal precessional centrifuge, the horizontal type makes the movement of the drum more complicated than verticle type. But we did our design on the basis of the mechanical model of "the stability conditions of rotation of a rigid body about a fixed point" and so obtained the stable conditions and parameters of the drum. Theoretical analysis proved that dynamic charactaristic of precessional centrifuge is similar to that of the centrifuge, which is rotating about one single fixed axis with almost the same size of the drum. The structure characteristics will be analysed and has been proved as follows: 1) The productive capacity of precessional centrifuge with low separationfactor is similar to that of the other type of the centrifuge which needs a much high separation factor. 2) The energy distribution ratio K between solid and hollow shaft of the drum can be obtained as formula wherefl-angle between the rotating and revolutionary axis(solid and hollow shaft). I . In accordance with the mechanical model ofthe stability condition of the rigid body rotating about a fixed point",an important formula for steady work of drum has been obtaind and condition Jxi, &2-the rotating and revolutionary anglar velocities of the drum, Jx, Jy, J*-the moments of inertia of the drum with respect to the three principle axes X, Y, Z through the fixed point