Recently, due to the rapid development of WEB2.0, the use of social media is becoming more important for decision makers. The aim of this research is to study the critical factors of user acceptance in the context of social media on Web-based decision support systems. In a combination of social media viewpoint and the consideration of decision support systems advantages, the critical factors that may impact on user acceptance are investigated. In line with the decomposed theory of planned behavior, this study constructs a hypothesized model for testing and verification. A well-designed Web survey was conducted. A total of 421 questionnaires were collected from online platforms such as LinkedIn, Facebook, and forum PPT, generating 308 usable replies with a total response rate of 73.2 percent. The results indicated that playfulness does not significantly affect user attitude toward acceptance of social media decision support system. The critical factors, which affect user attitude, include social interaction, task complexity, perceived risk, trust, and sense of belonging. Additionally, the eWOM, interpersonal influence and external influence that significantly affects subjective norms, and the Self-efficacy and facilitating conditions significantly affect perceived behavioral control and significantly affects user actual use. Through the empirical results, this study provides implications for the system developers, users, government, and the owners of social media decisions support systems.