Covalent CDK7 inhibitor THZ1 is a newly discovered anti-tumor drug.THZ1 affects the function of transcription factor TFIIH by inhibiting CDK7, which in turn affects RNA polymerase II, and ultimately affects transcription initiation. Study found that THZ1 could inhibit proliferation and promote apoptosis of several tumor cell lines. However, there is no report of the potential side effect of THZ1 in normal tissues. In the course of cancer, the muscle consumption of cachexia needs to be supplemented by the differentiation of muscle cells. However, the effect of THZ1 on myogenic differentiation remains unclear. Our study in this article found that THZ1 could both inhibit the differentiation of C2C12 cells and mouse primary myoblasts, also repressing the expression of differentiation-related transcription factors and muscle structural proteins, such as and myogenin, myh3 and MCK. Moreover, THZ1 could inhibit C2C12 cell proliferation and migration, increase its oxidative stress and promote its apoptosis. Our data indicates that THZ1 inhibits myogenic differentiation, suggesting that therapies based on THZ1 might have potential side effects on muscle functions.