A PLC testing system, comprising: an industrial personal computer on which a virtual device and a testing tool are deployed; and a PLC device; the virtual device simulating a field real device; the testing tool configured to start a preset test case to inject a fault into the virtual device; and the PLC device configured to execute a preset fault processing logic when receiving a fault signal produced by the virtual device simulating a field fault according to the injected fault, the preset fault processing logic comprising sending alarm information to the virtual device; wherein the virtual device is configured to forward the alarm information to the testing tool; the testing tool is further configured to detect whether the alarm information meets the expectation of injecting the fault by the preset test case to test whether the fault processing logic of the PLC device is normal; characterized in that: the testing tool is further configured to send a switching-to-manual signal simulating a field manual intervention action to the virtual device after detecting whether the alarm information meets the expectation of injecting the fault of the preset test case; the virtual device is configured to forward the switching-to-manual signal to the PLC device; and correspondingly, the PLC device is further configured to control the virtual device to switch to a manual mode when receiving the switching-to-manual signal; wherein the testing tool is further configured to send a reset signal simulating a field manual intervention action to the virtual device after the PLC device controls the virtual device to switch to a manual mode; the virtual device is configured to forward the reset signal to PLC device; and correspondingly, the PLC device is further configured to reset and clear the preset fault processing logic and control the virtual device to clear the alarm information when receiving the reset signal. an industrial personal computer on which a virtual device and a testing tool are deployed; and a PLC device; the virtual device simulating a field real device; the testing tool configured to start a preset test case to inject a fault into the virtual device; and the PLC device configured to execute a preset fault processing logic when receiving a fault signal produced by the virtual device simulating a field fault according to the injected fault, the preset fault processing logic comprising sending alarm information to the virtual device; wherein the virtual device is configured to forward the alarm information to the testing tool; the testing tool is further configured to detect whether the alarm information meets the expectation of injecting the fault by the preset test case to test whether the fault processing logic of the PLC device is normal; characterized in that: the testing tool is further configured to send a switching-to-manual signal simulating a field manual intervention action to the virtual device after detecting whether the alarm information meets the expectation of injecting the fault of the preset test case; the virtual device is configured to forward the switching-to-manual signal to the PLC device; and correspondingly, the PLC device is further configured to control the virtual device to switch to a manual mode when receiving the switching-to-manual signal; wherein the testing tool is further configured to send a reset signal simulating a field manual intervention action to the virtual device after the PLC device controls the virtual device to switch to a manual mode; the virtual device is configured to forward the reset signal to PLC device; and correspondingly, the PLC device is further configured to reset and clear the preset fault processing logic and control the virtual device to clear the alarm information when receiving the reset signal. the testing tool is further configured to send a switching-to-manual signal simulating a field manual intervention action to the virtual device after detecting whether the alarm information meets the expectation of injecting the fault of the preset test case; the virtual device is configured to forward the switching-to-manual signal to the PLC device; and correspondingly, the PLC device is further configured to control the virtual device to switch to a manual mode when receiving the switching-to-manual signal; wherein the testing tool is further configured to send a reset signal simulating a field manual intervention action to the virtual device after the PLC device controls the virtual device to switch to a manual mode; the virtual device is configured to forward the reset signal to PLC device; and correspondingly, the PLC device is further configured to reset and clear the preset fault processing logic and control the virtual device to clear the alarm information when receiving the reset signal.