Objective To study the chronological changes in myocardial fibrosis in spontaneously hypertensive rats. Methods Forty-five male spontaneously hypertensive rats (SHR) and 35 male Wistar Kyoto rats (WKY) that were four weeks old and specific-pathogen-free were randomly selected and fed in cages until 42 weeks. Systolic blood pressure was measured by the tail-cuff method , and cardiac structure and function were detected by echocardiography at pre-set time points. At the same time points, heart specimens were dissected, and the left ventricular mass index (LVMI) was calculated. Myocardial fibrosis of the left ventricle was detected by Masson staining, and TGF-β1, collagen I and collagen III were detected by qRT-PCR. Results The blood pressure of SHR increased rapidly from 6 weeks to 12 weeks and was significantly higher than that of WKY rats at 12 weeks (P< 0.05) and then stabilized and remained high. With age and blood pressure changes, the structure-related indexes of the left ventricle, including interventricular septal thickness (IVST), left ventricular posterior wall thickness (LVPWT) and LVMI, all showed dynamic characteristic alterations. The collagen volume fraction (CVF) gradually increased with the age of SHR and was significantly higher than that of WKY rats at 12 weeks and later ( P< 0.05). The expression of the fibrosis-related factors TGF-β1, type I collagen and type III collagen in the left ventricle myocardium of SHR all decreased at first and then increased with age to statistically significant levels after 32 weeks. Conclusions With increasing age and blood pressure, myocardial fibrosis in SHR continuously develops and presents with the typical characteristics of chronological changes, including structure, function, macro and micro alterations.