Abstract: Objective To explore the effect of smoking on the wound healing of stage 4 pressure ulcers in rats. Methods Fifty male Sprague-Dawley rats aged 6-8 weeks were divided into simple pressure ulcer group and smoking+ pressure ulcer group according to the random number table, with 25 rats in each group. After the rats in the smoking+ pressure ulcer group received passive smoking intervention for 12 weeks, an iron plate was placed in the back muscle of each rat in 2 groups, and a magnet was placed outside the skin at the corresponding position of the iron plate for 2 h at each time, with 5 times a day and continuously for 6 days to reproduce stage 4 pressure ulcer model. Immediately after establishing the model, 3 rats in each group were sacrificed and wound tissue was collected, and hematoxylin-eosin staining was applied to observe the pathological changes of the wounds. On 1, 3, 7, and 14 day (s) after establishing the model, 3 rats in each group were collected to measure the pressure ulcer wound area by the paper jam method. After measurement of the wound area, the rats were sacrificed and the wound tissue was collected, and the protein expression levels of matrix metalloproteinases 9 (MMP-9) and tissue inhibitor of metalloproteinase 1 (TIMP-1) in wound tissue were detected by immunohistochemical method, and the ratio of MMP-9/TIMP-1 was calculated.The wound healing time of the remaining 10 rats in each group was recorded. Data were statistically analyzed with analysis of variance for factorial design, two independent sample t test, and Bonferroni correction. Results (1) Immediately after establishing the model, muscle fiber necrosis and dissolution with large areas were seen on the wound, the myofibrils arranged loosely, and more lymphocytes and monocytes infiltration were seen around the wound of rats in simple pressure ulcer group. A large number of necrotic myofibers were dissolved and gradually disappeared, the myofibrils arranged loosely, and the number of diffuse lymphocytes and monocyte infiltration in wound of rats in smoking+ pressure ulcer group were significantly higher than those in simple pressure ulcer group. (2) The wound areas of rats in smoking+ pressure ulcer group were significantly larger than those in simple pressure ulcer group on 1, 3, 7, and 14 day (s) after establishing the model (t=3.019, 2.549, 2.181, 3.674, P<0.05 or P<0.01). (3) On 1 to 14 days after establishing the model, the protein expression levels of MMP-9 and TIMP-1 in the wound tissue and the ratio of MMP-9/TIMP-1 of rats in the two groups increased first and then decreased. On 1, 3, 7, and 14 day (s) after establishing the model, the protein expression levels of MMP-9 in the wound tissue and the ratio of MMP-9/TIMP-1 of rats in smoking+ pressure ulcer group were significantly higher than those in simple pressure ulcer group (t=4.783, 4.508, 6.325, 7.204, 3.078, 2.989, 4.081, 4.696, P<0.05 or P<0.01), and the protein expression levels of TIMP-1 in wound tissue of rats in the two groups were similar. (4) The wound healing time of rats in smoking+ pressure ulcer group was (48.9±2.6) d, which was significantly longer than (35.2±2.3) d of simple pressure ulcer group (t=12.477, P<0.05). Conclusions Smoking can up-regulate the expression of MMP-9 in pressure ulcer wound and result in an imbalance of MMP-9/TIMP-1, thereby affecting the wound healing of stage 4 pressure ulcers in rats.