While teaching assistants (TAs) have experienced various employment challenges, little research has been done to examine factors affecting their well-being. Based on an extended Control-Value model in studying teachers' emotions and well-being, the present study examined the relationship between primary school TAs' Control-Value variables in the context of formative feedback practice (abbreviated as TFBCV), their emotional well-being, and other well-being indicators. 305 TAs from 184 primary schools in Hong Kong participated in this survey study. Usually, there is at least one TA to support students with special educational needs in each mainstream school in Hong Kong. This sample represents 39% of primary schools in Hong Kong given that each school has one TA to join this study and these participating schools. The results supported our hypotheses that TFBCV has significant direct effects on both a combined positive emotion and the two discrete positive emotions (Joy and Love). TFBCV also has a significant indirect effect on job satisfaction via positive emotions. Interestingly, job stress was nearly uncorrelated with TFBCV and positive emotions. We discuss implications for extending teacher well-being research to include the 'missing' but essential piece of TAs in the current literature.