Abstract: An online tracer-tagged method coupled into a chemical transport model was described. Without the errors caused by the non-linear character of O3 photochemistry, the online tracer-tagged method can be used to estimate the contributions from the photochemistry over various regions to the total O3 concentrations in the targeted region in one simulation. To evaluate the results, taking the contributions from the photochemistry over the tagged northern and southern parts of China as an example, the online tracer-tagged method was compared with the revised sensitivity analysis. The comparison suggested that the online tracer-tagged method can reproduce the horizontal and vertical distributions of the contributions from two tagged regions to the O3 concentrations in East Asia, and also capture the typical heavy regional transport episode. This indicated the tracer-tagged method in this study can be used to quantitatively assess the impact of the regional transport on the tropospheric O3. The analysis also showed that the photochemistry in the southern and northern parts of China had a noticeable impact on O3 in East Asia, and varied with the seasons.