Background Sodium tanshinone IIA sulfonate (STS) has been widely used by Chinese medicine practitioners for chronic cardiovascular diseases. However, its direct clinical efficacy in patients with acute coronary syndrome following percutaneous coronary intervention (PCI) has not been reported yet. The present trial aimed to investigate potential cardioprotection of STS in patients undergoing PCI for non-ST elevation acute coronary syndrome (NSTE-ACS). Methods In a randomized, double-blind, placebo-controlled trial, 372 patients with NSTE-ACS were randomly assigned to receive STS (n = 192) or saline (n = 180) for 2 days before and 3 days after PCI along with standard therapy. The primary endpoint was the composite incidence of major adverse cardiac events (MACEs), including death, non-fatal myocardial infarction, repeated revascularization of the target vessel, and stent thrombosis, within 30 days after PCI. Results The 30-day MACEs occurred in 18.8% of the patients in the STS group and in 27.2% of the patients in the control group (P = 0.038); this difference was mostly driven by reduction of myocardial infarction incidence (17.2% vs. 26.7%,P = 0.027). Post-procedural elevation of troponin-I was also significantly lower in the STS group (26.56% vs. 47.78%,P < 0.001). Multivariable analysis identified STS as a predictor of decreased risk of MACE occurrence (odds ratio: 0.60, 95% confidence interval: 0.36 to 0.99;P = 0.045). Conclusion Addition of STS to the standard treatments recommended by the current practice guidelines in patients with NSTE-ACS undergoing PCI could reduce myocardial injury and the occurrence of short-term cardiovascular events, primarily driven by non-fatal myocardial infarction.