Promoting undergraduates computational thinking (CT) remains a significant challenge. The purpose of this study was to foster students’ CT in a knowledge building (KB) environment. The participants were 49 college students from a Chinese university. They were randomly assigned to ten groups. Each group used Knowledge Forum (a computer-supported collaborative knowledge-building environment), supported by customized scaffolding, to discuss and develop the computational artifacts of their choice. Data were collected from student groups’ online discourse, pre-post CT scale, and group artifacts. We employed Friedman test, Wilcoxon signed rankings test, and Kruskal-Wallis test to analysis data. In general, we found KB activities facilitated students’ CT. Engaging in KB activities can significantly promote students’ algorithmic thinking, cooperativity, critical thinking, and problem-solving skills, except for their creativity. Moreover, students demonstrated differential engagement in various CT tasks, as reflected by their discourse and products, and needed more support in evaluation & generalization. This study can offer insights for educators to design KB environment to support students’ CT.