A computer system, comprising: a processor; and a peripheral component interconnect express, PCIe, bus, configured to connect to a PCIe endpoint device, wherein the processor is configured to obtain an operation instruction for accessing the PCIe endpoint device, and send an access request to the PCIe endpoint device according to the operation instruction; and the computer system further comprises a simulation response module, set in a signal transmission path between the processor and the PCIe endpoint device, is configured to receive and buffer the access request sent by the processor, monitor a message state of the buffered access request by monitoring whether a response message for the buffered access request is received from the PCIe endpoint device, and if a response message for the buffered access request is received from the PCIe endpoint device, delete the buffered access request, determine the message state of the buffered access request when a preset trigger condition is fulfilled, and if the message state of the buffered access request is unfinished, obtain a simulation response message for the buffered access request, and send the simulation response message to the processor, wherein the message state of the buffered access request comprises finished or unfinished, wherein unfinished indicates that the PCIe endpoint device sends no response message for the buffered access request, the message state of the buffered access request is finished if the buffered access request is deleted; wherein the preset trigger condition comprises that an interruption notification indicating that the PCIe endpoint device is plugged out of the system is received or a time-out of a timer of the access request. a processor; and a peripheral component interconnect express, PCIe, bus, configured to connect to a PCIe endpoint device, wherein the processor is configured to obtain an operation instruction for accessing the PCIe endpoint device, and send an access request to the PCIe endpoint device according to the operation instruction; and the computer system further comprises a simulation response module, set in a signal transmission path between the processor and the PCIe endpoint device, is configured to receive and buffer the access request sent by the processor, monitor a message state of the buffered access request by monitoring whether a response message for the buffered access request is received from the PCIe endpoint device, and if a response message for the buffered access request is received from the PCIe endpoint device, delete the buffered access request, determine the message state of the buffered access request when a preset trigger condition is fulfilled, and if the message state of the buffered access request is unfinished, obtain a simulation response message for the buffered access request, and send the simulation response message to the processor, wherein the message state of the buffered access request comprises finished or unfinished, wherein unfinished indicates that the PCIe endpoint device sends no response message for the buffered access request, the message state of the buffered access request is finished if the buffered access request is deleted; wherein the preset trigger condition comprises that an interruption notification indicating that the PCIe endpoint device is plugged out of the system is received or a time-out of a timer of the access request.