After the outbreak of Corona Virus Disease 2019(COVID-19), multiple measures have been implemented to slow down the spread of virus, including travel restrictions and stay-at-home order, since the Chinese New Year 2020, leading to a huge reduction of the primary emission. However, several air pollution events still occurred in the Sichuan Basin. By combining in-situ observation and WRF-Chem simulations, this work aims to explore the cause of the air pollution during the lockdown period in the Sichuan Basin. Due to substantial transportation emission reduction, NO₂ concentration was significantly reduced from 35.0 μg·m^-3 to 20.2 μg·m^-3 in the Sichuan Basin area, dropping about 40%-60%. Spatially, there existed regional disparities in the change of O₃ concentration because of the NO_X-VOCs-O₃ non-linear relationship and the unbalanced emission reduction of NO_X and VOCs in the Sichuan Basin. During the lockdown,O₃ concentration in this region increased from 27.5 μg·m^-3 to 41.4 μg·m^-3 with the maximum increase reached 100% in Chongqing, and it enhanced secondary aerosol formation by increasing the atmospheric oxidation capacity. In the Chengdu and Chongqing area, the increasing of O₃ and atmospheric oxidation capacity promoted secondary aerosol formation, which somehow counteracted the primary pollution reduction. Meanwhile, in the other area of the Sichuan Basin, a decreasing O₃ and atmospheric oxidation capacity suppressed the secondary formation, and the mean PM_2.5 concentration reduced from 57.0 μg·m^-3 to 47.4 μg·m^-3. The rate of secondary and primary pollution changed about-10% to 20% in Chengdu and Chongqing area, and it decreased 30%-60% in the other area of the Sichuan Basin. In conclusion, the unbalanced primary emission reduction and the increasing atmospheric oxidation capacity favored the secondary pollution formation, which highlights that air quality control in the Sichuan Basin requires a coordinated and balanced control of multiple pollutants.