Far-reaching effects of positive affect in single task and multitask environments need to be reflected in human performance modeling to develop higher fidelity models. The present study focuses on the effects in the multitasking environment. Specifically, we developed the ACT-R cognitive model to identify the cognitive mechanisms responsible for decrements of switching cost by positive affect in the Dimensional Change Card Sort (DCCS) task. The effect of positive affect on switching cost on the DCCS task relies heavily on the declarative knowledge of actionrepertoires matched to a displayed cue, allowing us to justify existing mechanisms of positive affect from previous research. Compared to the experimental data, the results show that simulated data of the ACT-R cognitive model achieved by the proposed method were highly matched with the average response time from the experimental data. The present study contributes to a general account of how positive affect impacts human cognitive process.