Abstract: In order to solve the cracking of powder metallurgy （PM） superalloy turbine disc during isothermal forging it is necessary to use a special process of isothermal forging at constant strain rate. The displacement expression of PM billets during deformation with constant strain rate and the oil pouring equation of oil supplying system have been derived in this paper. Considering the real operation conditon of forging machine a dispersing treatment has been made to oil pouring equation to meet the needs of deformation of billets at constant strain rate.