As the computer's use of people's cognitive load and mental stress will help the achievement of smart life in the future, meanwhile recent studies show that the PPG signal which reflects the changes of blood volume in the microvascular will help to analyze the mental stress and emotion measurement. The paper introduces a novel photoplethysmogram-based stress induced vascular index (sVRI) to measure cognitive load and stress by using a reflective PPG signal sensor. This model demonstrates the applicability of the reflective PPG signal sensor. The experiment in this paper does not only compare the waveform difference of using the transmission mode and reflective mode to acquire the PPG signal, it also uses the reflective green light compared with the red and infrared light. The mental stress is evaluated based on reflective photoplethysmogram. Baed on the comparison and discussion, the applicability of the reflective PPG signal sensor is proved. At last, the paper applies the results of the experiment to the model of the mouse. It has a great influence on the individual health problems in daily life and the advantage of comfort for the individuals and the mobility convenience can expand considerably the range of wearable smart devices and high-performance medical care devices used for pulse detection system.