A bridging domain method,in which molecular models and continuum models are coupled,is performed to study threedimensional contact of a rigid spherical tip and a smooth fiat substrate.It is compared with full-atom molecular simulations.We focus on relaxation behavior under a given load,continuity of displacement and stress at bridging domain,normal force and contact radius.It shows that the bridging domain method quickly reaches equilibrium and has weaker oscillations under a given external load.It has small relative errors as initial temperature of system is at 0 K.The bridging domain method can transmit continuously displacement and stress from molecular domain to continuum domain,and has goad coupling effect.Normal force-displacement and contact radius-displacement are nearly coincide with molecular simulations.Its computational accuracy is high.