The coronavirus pandemic has brought public health challenges worldwide. The main protease (Mpro), also called 3CLpro (3C-like protease), is encoded by the gene of non-structural protein 5 (nsp5). It is an attractive antiviral drug target to halt the progress of sudden accuse respiratory syndrome coronavirus 2 (SARS-COV-2), the causative pathogen of coronavirus disease 2019 (COVID-19) [1]. After viral infection in host cells, the replicase gene encodes two polyproteins, pp1a (486 kDa) and pp1ab (790 kDa). They are then cleaved by papain-like protease 2 (PL2pro) at 3 sites and Mpro at another 11 sites. Then, pp1a and pp1ab are processed to release a series of non-structural proteins (NSPs) that mediate viral replication and transcription [1]. The function of Mpro is indispensable to the viral life cycle. Thus, inhibiting the activity of Mpro can effectively impede the coronavirus replication. Over one 100 host-cell proteins that are involved in essential cellular processes have also been reported as substrates of SARS-COV-2 Mpro, suggesting that Mpro is a multifunctional viral factor [2]. To evaluate drugs that may target SARS-COV-2 Mpro for COVID-19 treatment, we purified SARS-CoV-2 Mpro protein and attempted to validate certain compounds that have been reported to inhibit coronavirus Mpro effectively [3]. The SARS-COV-2 Mpro was expressed in Escherichia coli cells and then purified via affinity and size-exclusion chromatography. The purified SARS-COV-2 Mpro showed a monodispersed peak with a molecular weight of ~33.8 kDa (Supplementary information, Fig. S1a). We selected nine drugs and performed thermal shift assay (TSA) to assess the effect of each drug on SARS-CoV-2 Mpro. Each drug (20 μM) was incubated with SARS-COV-2 Mpro and the melting profile was monitored based on the SYBRO orange reaction in the range of 25–99 °C. Only GC376, an antiviral drug used to treat feline coronavirus disease [4], displayed an obvious effect, shifting the melting curve of SARS-CoV-2 Mpro from 50.9 °C to 55.2 °C (+4.3 °C). Other drugs showed little effect (Fig. 1a, Supplementary information, Table S2). These results show that GC376 may bind to SARS-COV-2 Mpro. To determine if the process is titratable, the TSA experiment was repeated with different concentrations of GC376 (0–20 μM). The melting temperature of Mpro was found to increase in a dose-dependent manner (Fig. 1b). Taken together, our results showed that GC376 interacts directly with SARS-COV-2 Mpro.