Objective: To observe clinical efficacy of fractional flow reserve (FFR)-guided percutaneous coronary intervention (PCI) in coronary heart disease patients with SYNTAX scores (SS) ≥33 and Euro Scores (ES) ≥6 who are unsuitable for or have declined coronary artery bypass graft (CABG). Methods: A total of 117 patients with SS ≥33 and Euro Score (ES) ≥6 who were unsuitable for and/or who had declined CABG between Jan 2021 and June 2022 were enrolled in this retrospective analysis. All patients accepted optimal medical therapy and some accepted an FFR-guided PCI procedure. Patients who only underwent optimal medical therapy were divided into the optimal medical therapy group (OMT group) and patients who simultaneously underwent FFR-guided PCI procedure were divided into the PCI group in this retrospective analysis. All patients accepted follow-up for at least 12 months after discharge. Results: SS and ES in the two groups were not statistically different (p > 0.05). Patients with chronic total occlusion accounted for a greater proportion in the PCI subgroup (31.3%, 5/16) than in other subgroups. Eighteen (18.6%, 18/97) cases in the PCI group developed major adverse cardiac and cerebrovascular events (MACCEs). There were 12 (60%, 12/20) cases of MACCEs in the OMT group, which was statistically different from the PCI group (p < 0.05). Conclusions: Based on optimal medical therapy, FFR-guided PCI can still have clinical benefit to coronary artery disease patients with SS ≥33 who were not suitable for CABG.