The dynamic, adiabatic and diabatic entrance potentials in strongly damped reactions of 238 U+238 U, 232Th+250 Cf are calculated and compared. The feature of the dynamical potential implies that it is possible for the composite systems to stick together for a period of time. By means of the improved quantum molecular dynamics model the time evolution of the density and charge distributions of giant composite systems and their fragments for reactions 238U+238U, 232Th+250Cf are investigated, from which the lifetimes of giant composite systems at different energies are obtained. The longest average lifetime of 238 U+238 U is found when the incident energy is about Ec.m.=1080 Me V, which is about 1200 fm/c.