The coupled and uncoupled Langevin equations in two-dimensional collective space are used to study the neck dynamics of the mass asymmetric system 48Ca+238U at the above barrier energy.The results show that the coupling between the neck and radial degrees of freedom delays the transition from dinucleus to mononucleus,and correspondingly increases the lifetime of the dinucleus system.The lifetime of the dinucleus for the asymmetric system 48 Ca+238 U is about 11.6 and 13.0 × 10-22 s,obtained with uncoupled and coupled Langevin equations.We caculate the evaporation residue (ER) cross sections for the 3n and 4n evaporation channels in the 4s Ca+238U reaction leading to the formation of 283112 and 282112 isotopes in three different approaches,i.e.,coupled,uncoupled and frozen approximation,and compare them with the experimental data.It is found that the results of the uncoupled and frozen approximation are in dose similarity,while the coupling between the radial and neck degrees of freedom reduces the ER cross section by about 30％,compared with the case of frozen approximation.