The invention relates to a fault simulation device of a numerical control machine. In the fault simulation device, a field bus and the numerical control device are connected and the fault simulation device is internally provided with an upper computer and a lower computer. The lower computer is an industrial personal computer (PLC) with a Linux real-time operating system, and is mainly in charge of operating the PLC logical model of the numerical control machine and exchanging data with the numerical control device through the field bus; and the upper computer is a personal computer (PC) with a WINDOWS operating system, is connected with the lower computer by the Ethernet and is mainly used for solving man-machine interaction operations such as numerical control machine parameter setting, fault setting, fault alarm and the like. The fault simulation device disclosed by the invention is mainly used for solving test conditions of the failure detection function and capacity of top grade numerical control devices and simulating numerical control machine faults, programmable logic controller faults, servo driver and servo motor faults, and main shaft driver and main shaft motor faults. The fault simulation device of the numerical control machine can be used for detecting capacities of the numerical control device such as response, protection and auto recovery on the faults of the numerical control machine.