A high-fidelity simulator for a construction machine particularly contributes to an unmanned construction system since the simulator efficiently manages and schedules tasks required for the machines in a systematic way. Besides, the simulator also provides a virtual reality as a training platform for an operator for tele-operated unmanned construction machine. The simulator is generally subject to a well-known trade-off between computational burden and simulation accuracy. Therefore, the main scope of this research is to establish an accurate and computationally efficient simulator for an excavator, and particularly this paper focuses on an interaction model of an excavator bucket. An accurate excavation mechanics is of a key issue to characterize dynamic motion of the excavator. The model developed in this paper basically exploits the Resistive Force Theory with considering soil surcharge while excavating. The proposed model is experimentally validated in different types of excavation motion using a bucket experimental apparatus.