According to the causes of death for people in 2018 provided by the Ministry of Health and Welfare in Taiwan (R.O.C), breast cancer and oral cancers rank 4 th and 5 th respectively among the top 10 cancer death rates. In order to avoid affecting the appearance or basic function of the defect site after the tumor resection operation, free skin flap surgery have to be performed. As the success rate of the surgery is currently around 97%, there are still 3% failures. Although the probability of failure is relatively low, once the flap becomes necrotic, one or two larger operations have to be performed for 7 to 8 hours. Therefore, monitoring after free skin flap transplantation to avoid skin flap necrosis is important. The monitoring of free flaps often relies on the experience of medical staff or professional equipment, and it is difficult to find problems in time. In addition, if the medical staff is inexperienced, it is easy to misjudge. This study cooperated with Tzu Chi Hospital in Hualien, Taiwan to provide a non-contact image analysis of heart rate as a method to decide whether the flap is necrotic, which can avoid infection caused by contact with the flap. The Difference-driven Weight-Setting algorithm based on the principle of Photoplethysmography (PPG) is used to analyze the signal to obtain the heart rate. The low time complexity of our algorithm shows the feasibility to utilize embedded systems for measurement. The experimental samples were collected from 36 cases after free skin flap surgery, and the heart rate measured by electrocardiograph (ECG) was compared with the heart rate calculated via the image with our proposed mechanisms. The initial analysis of the image is divided into two kinds of image quality: 1080p (1920 × 1080 pixels) with an accuracy rate of 71% and 720p (1280 × 720 pixels) with an accuracy rate of 36%, lower than expected. In the study, the factors affecting the measurement were investigated and improved. Further analysis of the stable image recorded at 720p was subsequently performed. The accuracy rate had been improved to 73% and the measurement time had also been substantially reduced by 36%.