Extravasation injury is caused by the efflux of solutions from a vessel into surrounding tissue spaces during intravenous infusion and cannot be observed on the skin easily. When the drug exuding from the blood vessel spreads to the subcutaneous tissue and the problem is not dealt with promptly, it not only causes discomfort and pain, but could also result in tissue necrosis or even amputation. This study proposed a novel wearable device for extravasation monitoring based on array sensor with Near-infrared spectroscopy (NIRS) to detect the abnormal changes of fluid volume in tissue, such as blood and drug. The array sensor was composed of four 760 nm transmitters and processed with time division method in every 1.2s. The results indicated that the proposed method could successfully distinguish the subcutaneous region of extravasation with high sensitivity and could be monitored on the OLED screen. It also could be transmission of data to a PC via Wi-Fi for further real-time analysis. The design had the potential to be an assistant tool for related application on clinical care.