Aim To investigate the serum level of long non-coding RNA myocardial infarction-associated transcript (lncRNA MIAT) in patients with type 2 diabetes mellitus (T2DM) and coronary heart disease (CHD) and its effect on high glucose (HG)-induced myocardial cell injury. Methods From June 2021 to December 1,0 patients with uncomplicated T2DM who visited Tangshan People's Hospital were regarded as the T2DM group, 100 patients with uncomplicated coronary heart disease (CHD) were regarded as the CHD group, and 100 T2DM patients with CHD were selected as T2DM+CHD group, in addition, 100 healthy people were regarded as the control group. Real-time quantitative PCR (RT-qPCR) was applied to detect the levels of blood MIAT and microRNA-150-5p (miR-150-5p). The human cardiomyocyte line AC16 cells were cultured in vitro and grouped into NG group (5.5 mmol/L normal glucose), HG group (30 mmol/L high glucose), HG+MIAT knockdown negative control (HG+si-NC) group, HG+MIAT knockdown (HG+si-MIAT) group, HG+si-MIAT+miR-150-5p inhibitor negative control (HG+si-MIAT+anti-NC) group, and HG+si-MIAT+miR-150-5p inhibitor (HG+si-MIAT+anti-miR-150-5p) group. RT-qPCR was performed to detect the expression of MIAT and miR-150-5p in cells; MTT assay was performed to detect cell proliferation viability; Flow cytometry was performed to detect apoptosis; ELISA method was implemented to detect lactate dehydrogenase (LDH) content; Western blot was performed to detect protein expressions of cyclin D2 (CCND2),Bü cell-lymphoma-2 gene (Bcl-2), Bcl-2-associated X protein (Bax), and cysteine protease-3 (Caspase-3), cleaved Caspase-3; Dual-luciferase reporter, RNA-binding protein immunoprecipitation (RIP) and RNA pull down were performed to analyze the targeting relationship of miR-150-5p to MIAT and CCND2. Results Compared with control group, CHD group and T2DM group, the expression level of MIAT was obviously increased in T2DM+CHD group by 2.69 times, 1.71 times and 1.42 times (P＜0.05), and the expression of miR-150-5p was obviously decreased by 68.63%, 60.49% and 46.67% (P＜0.05). Correlation analysis showed that the expression level of MIAT were negatively correlated with miR-150-5p in T2DM+CHD patients (r=－0.662, P＜0.001). Compared with NG group, MIAT expression in AC16 cells was increased in HG group by 3.54 times, cell apoptosis rate, LDH activity, Bax protein level, and cleaved Caspase-3/Caspase-3 ratio were increased by 5.22 times, 2.19 times, 2.90 times, and 3.83 times, respectively; The expression of miR-150-5p was decreased by 75.00%, and the proliferative activity of cells at 4,8 and 72 h was decreased by 49.02%, 52.38%, 49.48%, and the protein levels of CCND2 and Bcl-2 were decreased by 72.62% and 78.26%, respectively (all P＜0.05). MIAT knockdown increased the expression of miR-150-5p by 3.46 times, alleviated HG-induced AC16 cell damage and reduced cell apoptosis by 65.73% (all P＜0.05); Inhibition of miR-150-5p significantly weakened the effect of MIAT knockdown on HG-induced AC16 cell damage (P＜0.05). MIAT targeted and negatively regulated miR-150-5p expression, and CCND2 was a target gene of miR-150-5p. Conclusion Serum MIAT level increased in T2DM patients with CHD. MIAT knockdown may antagonize HG-induced human cardiomyocyte injury by regulating miR-150-5p.