Compared with the traditional two-dimensional (2D) monolayer culture, three-dimensional (3D) organoid can better simulate the physiological and pathological microenvironment of organs and tissues. In this study, 3D cardiac organoids were constructed using cardiac fibroblasts (CFs), cardiac myocytes (CMs) and endothelial cells (ECs) isolated from hearts of 1-3-day Sprague-Dawley (SD) neonatal rats. The experimental scheme was approved by the Experimental Animal Welfare and Ethics Committee of Tianjin University of Traditional Chinese Medicine and met the standards of experimental animal welfare and ethics. Optimal seeding cell density and culture time were determined by observing the sphere diameter and pulsation. The hierarchical structure and cardiac-like function were evaluated by fluorescence staining. The results showed that the cardiac-like microspheres constructed with cell number of 1×10⁴ still beated spontaneously even after 34 days in culture, and maintained characteristic cellular hierarchical structure. Then, based on these cardiac microspheres, a phenylephrine (PE)-induced cardiac hypertrophy model was established and evaluated by mitochondrial mass, intracellular Ca²⁺ concentration and mitochondrial membrane potential. Guanxinning Injection (GXNI) was tested to verify that the established model can be used for myocardial hypertrophy drug screen. The results showed that GXNI significantly reversed the enlargement of cardiac microsphere area and diameter, the increase of mitochondrial mass, intracellular Ca²⁺ concentration and the decrease of mitochondrial membrane potential caused by PE, and reduced upregulation of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and β-myosin heavy chain (β-MHC). In conclusion, this study successfully established a 3D in vitro model of cardiac remodeling induced by cardiac hypertrophy. In this new system, cardiac microspheres not only have cardiac-like morphology and extracellular matrix components, but also exhibit spontaneous and rhythmic systolic and diastolic function. Therefore, the cardiac microsphere is an effective model to investigate the pathological mechanism of cardiac hypertrophy and screen related drugs.