The sealing flow will inevitably affect the cooling airflow quality in a second air system of an aero-engine. In this study, theoretical derivation and numerical simulation were conducted to establish a modified mixing model that considers both the torque and the source of the mixing flow. Then, the effect of the sealing flow can be evaluated using the modified mixing model. The results demonstrate that the effects of the sealing outflow on the flow and temperature characteristics of the pre-swirl system are weak when the air supply mass flow rate and pressure are fixed. However, the effects of the sealing inflow are significant. When the inner sealing inflow mass flow rate is increased from 0 to 20%, the temperature drop effectiveness is decreased by 31.3%. The temperature drop effectiveness is decreased by 29.2% as the inner sealing inflow temperature rises by 37 K. Then, the temperature drop effectiveness is increased by 15.6% as the inner sealing inflow swirl ratio is increased from 0 to 0.8. The results of the modified mixing model are in satisfactory agreement with the numerical results, which show a maximum temperature drop deviation of 7.22%. Compared with the previous method, the prediction accuracy of the pre-swirl cavity temperature drop can be increased by 22.28%.