Two main characteristics of grid model in data of on line dynamic security assessment system for bulk power system are pointed out, which include loads hanging on 220 kV buses and equating generators, in which booster transformer’s highest voltage level is 110 kV, (110 kV generators for short), into load of constant impedance. Their impacts on the following analysis and calculation are expounded and proved. It is found that the dynamic stability of large disturbances varies with the fault position and grid's status, the oscillation mode related to the 110 kV generators is eliminated in the small signal stability analysis, and the calculation value of short-circuit current is smaller based on the style of case, but for the calculation value of short-circuit current based on the power flow, it is larger due to loads equivalents while smaller due to generations equivalents. The conclusions are validated by operation data of Huazhong grid with Power System Analysis Software Package (PSASP).