Although perceived risk and usefulness have been identified as two major factors that influence the acceptance of an innovative mobile payment, relatively few researchers have explored the impact of affective factors on user-perceived risk and usefulness. We also do not know whether there is a difference in the acceptance intention among users across different diffusion stages of this innovation. This paper aims to investigate: (1) the relative effects of cognitive (perceived risk and perceived usefulness) and affective (positive emotion) factors on acceptance intention, (2) the role of positive emotion in decreasing perceived risk and increasing perceived usefulness, and (3) the moderating effects of diffusion stages (introduction vs. growth) on the relationships between these three determinants and users’ intentions to accept a WeChat payment, an innovative mobile payment that has various usage scenarios in China. Empirical analysis on two datasets show that users’ acceptance intention is relatively related to perceived risk, perceived usefulness, and positive emotion. Positive emotion has a strong negative impact on perceived risk and a positive impact on perceived usefulness. More importantly, multi-group analyses find that both positive emotion and perceived risk have significant positive and negative impacts on acceptance intention at the stage of market introduction rather than market growth. Rather, the influence of positive usefulness on acceptance intention is significantly higher at the stage of market growth than at market introduction. The findings of this study indicate that exploring the moderating effect of diffusion stages in mobile payment acceptance provides a more comprehensive understanding of how to achieve a greater acceptance rate of an emerging or innovative mobile payment service.