Mechanic-electric simulation of a real receiving-end power system shows that the load dynamics have a significant impact on the symmetrical three-phase short-circuit current within 50 ms after short-circuit. This is supported by the fault-recording data of a power grid accident. The simulation data and the fault-recording data both indicate that the result from the traditional practical short-circuit calculation can be used to check the interrupting capacity of the circuit breaker， but can not be used to check the stress intensity of the circuit breaker.