Social media technology and social network facilitate the value creation of social commerce． It is a key question for social commerce companies to improve the purchase conversion efficiency by learning how to effectively harness the power of technology and social tie． However，few studies have explored how these two factors influence users’purchase decision． To address this gap，this paper constructs a theoretical research model to explore the social purchase behavior mechanism based on S-O-Ｒ framework by integrating technology affordance as well as strong and weak tie as stimulus factors，diagnosticity and serendipity as organic factors， and social commerce purchase as the response factor． This study collected data from users with social commerce purchase experience on WeChat，and examined the hypothesis by using Smart PLS 3.0.The results found that: technology affordance and weak tie have significantly positive effects on both diagnosticity and serendipity; the effect of weak tie on serendipity is larger than that of strong tie on serendipity; diagnosticity positively influences serendipity; both diagnosticity and serendipity have positive effects on social commerce purchase intention． However，this study also found that: strong tie has no significant effect on serendipity; diagnosticity and serendipity play a partial mediating role on the relationship between technology affordance and purchase intention and play a full mediating effect on the relationship between weak tie and purchase intention; diagnosticity plays a partial mediating role on the relationship between strong tie and purchase intention． This study extends research on social commerce and provides theoretical foundation and practical guidance for researchers and practitioners．