he forging process is a problem concerning large plastic deformation, so that this paper adoptedthe comoving coordinate to follow the tracks of flowing particls, and used the energy method to calculate, stepby step, increment of displacement, effective strain field, etc.

Based on the geometrical nonlinear theory of system consisted of bars, the formula of nodal displacement increment really causing the deformation of spatial beam element in its motion from t to t+Δt moment is derived in spatial co-rotational coordinate system.

Trough the analyseing the principle of coordinate system conversion and the principle of conductor measure conclusion, leading the calculation formula of the whole curve s point and the crossing point of tangent parallel lines with vertical parallel lines under the same coordinate system.