Objective Cardiac fibroblasts account for more than 50% of heart cells and play an important role in the physiological and pathological processes of the heart. Seven days of age is an important turning point in mice when the heart loses its ability to regenerate. This study analyzed the changes in the number and characteristics of cardiac fibroblasts in the early postnatal period of mice, and provides basic experimental data for the study of the myocardial regeneration mechanism. Methods Apical resection (AR) was performed on 1-day-old and 7-day-old C57 mice. The apex incision was photographed with a stereoscope and stained with hematoxylin and eosin (HE), and regeneration was observed at 21 days post resection. Heart tissues of C57 mice at postnatal day 1 (P1) and P7 were isolated and weighed to compare the wet weight. Cardiac pathological structures at the two time points were observed by HE staining in pathological sections, and fibroblasts were observed by immunofluorescence staining. To accurately quantify the number of cardiac fibroblasts, we used fibroblast surface marker thymocyte differentiation antigen 1 (Thy1) to sort cardiac tissue cell suspensions by flow sorting technology. To investigate changes in cardiac fibroblast characteristics, we isolated mouse cardiac fibroblasts on P1 and P7 and analyzed the mRNA expression of fibroblast marker proteins Thy1, fibroblast-specific protein 1 (Fsp-1), Periostin, platelet-derived growth factor receptor α ( Pdgfrα), collagen alpha-1 ( Col1a1), and transcription factor 21 ( Tcf21). Results Heart tissue of 1-day-old mice could regenerate (regeneration rate was 86. 67%) 21 days after apical resection, whereas that of 7-day-old mice could not regenerate ( regeneration rate was 0). The average wet weight of heart tissue in mice at P7 was 13. 13 mg (3. 11 times) heavier than that at P1 (P1: 6. 22 ± 0. 19, P7: 19. 35 ± 0. 56, P< 0. 0001; n= 6). Immunofluorescence staining indicated that the number of cardiac fibroblasts at P7 was significantly higher than that at P1. Flow cytometry showed that the proportion of cardiac fibroblasts to total cells in heart tissue at P7 was 2. 9% (1. 38 times) higher than that at P1 ( P1: 7. 7 ± 0. 74, P7: 10. 6 ± 0. 95, P= 0. 029; n= 3). At the level of fibroblast transcription, Thy1, Fsp-1, and Periostin expression was higher at P7 than at P1 (Thy1: P1: 1. 01 ± 0. 12, P7: 2. 71 ± 0. 27, P= 0. 0288; Fsp-1: P1: 1. 04 ± 0. 27, P7: 5. 28 ± 0. 10, P= 0. 0046; Periodin: P1: 0. 91 ± 0. 01, P7: 1. 13 ± 0. 05, P= 0. 0119; n= 3), and Pdgfrα, Col1a1, and Tcf21 expression was lower at P7 than at P1 (Pdgfrα: P1: 1. 09 ± 0. 04, P7: 0. 62 ± 0. 01, P= 0. 0068; Col1a1: P1: 1. 00 ± 0. 09, P7: 0. 57 ± 0. 02, P= 0. 0433; Tcf21: P1: 1. 00 ± 0. 03, P7: 0. 54 ± 0. 02, P= 0. 0054; n= 3), which indicated that cardiac fibroblast traits changed during the period from 1 to 7 days after birth. Conclusions The proportion and characteristics of cardiac fibroblasts in mice changed significantly during the early postnatal period (1 to 7 days), which provides some clues to the mechanism of the loss of mammalian myocardial regeneration.