Abstract: In order to study the effect of frequency reduction of the factional frequency transmission system on the initial amplitude of the oscillation torque generated by the three-phase short circuit in the synchronous generator port, a 5-order model of the generator is derived to resolve the analytical expression of the oscillatory torque. This paper compares the generator terminal voltage and power angle in both the power frequency system and the fractional frequency system with the same rated generator capacity, power factor, and operation state. Within the practical scope of standard parameters and electrical parameters of the synchronous generator, the monotonicity of the initial amplitude of the oscillation torque with respect to the terminal voltage and power angle is studied and the initial amplitudes of the oscillation torque of the fractional frequency system and the power frequency system are compared under the same condition, and the influence degrees of various factors on the initial amplitude of the oscillation torque are quantitatively analyzed and compared. The results show that the initial amplitude is smaller in the factional frequency transmission system when only the frequency reduction is considered, and the direct-axis sub-transient reactance has the greatest influence on the initial amplitude of the oscillation torque, while the quadrature-axis synchronous reactance has the least influence.