A venous reservoir (VR) is crucial for safe blood volume regulation during cardiopulmonary bypass (CPB) procedures. Currently, blood volume measurement in VRs relies heavily on visual inspection, which limits its accuracy and responsiveness. In this paper, a method that leverages a laser displacement sensor for blood surface distance change detection in VRs with high accuracy and simplicity is proposed. It enables precise, non-contact, real-time measurements for displacement and position analysis of blood surface. Hard-shell VRs maintain a constant relationship between blood volume and blood surface height. Therefore, we measured the blood surface distance and the distance between the sensors. The findings of this study demonstrate that blood volume correlates with the distance measured from the blood surface to the sensor, showing clear “blood surface distance-volume characteristic” curves specific to each VR. Experimental results confirm excellent reproducibility and low variability, even under different conditions, such as blood dilution and varying ambient light. In addition, the blood volume dynamic behavior during CPB can be captured with measurements that are in good agreement with theoretical predictions. Our findings confirm the potential of blood volume estimation based on blood surface distance measurements, making it a promising alternative for real-time blood volume monitoring during CPB. Integrating this method into CPB systems could improve safety, prevent air embolisms, and improve fluid management, paving the way for more precise and automated circulating blood volume control.