The increased Simon effect with increasing the ratio of congruent trials may be interpreted by both attention modulation and irrelevant stimulus-response (S-R) associations learning accounts, although the reversed Simon effect with increasing the ratio of incongruent trials provides evidence supporting the latter account. To investigate if learning irrelevant S-R associations is a common mechanism underlying the proportion congruency (PC) effect of the Simon task, we employed a variant of diffusion model, diffusion model for conflict tasks (DMC), to test which theory can simultaneously account for the mean reaction time (RT) and RT distribution patterns of the Simon effect in different PC conditions. Simulation results showed that the DMC modulating starting point according to learned irrelevant S-R associations rather than drift criterion or attention-related parameters (i.e., drift rate of controlled process, peak amplitude and time-to-peak of automatic activation) can simultaneously simulate the increase and reversal of the Simon effect and the different shapes of delta functions in different PC conditions. Moreover, when fitting to empirical data, the DMC adjusting starting point provided a good fit to the mean RT and RT distribution patterns of the Simon effect in different PC conditions. These results suggest learning irrelevant S-R associations (biasing starting point) may be a common mechanism underlying the PC effect of the Simon task.