Objectives To design blood level and pressure monitor,and evaluate its application during cardiopulmonary bypass in clinic.Methods The monitor system was composed of pressure transducer,photoelectric transmitter,photoelectric detector,core controller and liquid crystal display.The difference of refractions beween different materials was employed to develop the monitor as level dectector.The blood level in reservoir was determined by amplifying and quantifying the detected light signal,and the arterial line pressure was measured and converted to electricity signal by a pressure transducer.The pressure values of pressure monitor were compared with those of mechanical gauge.Finally we practically observed whether the monitor alarm under critical level and pressure condition.Results Positive correlation was demonstrated between the pressure readings and the pump flow. Significant differences were not found between the monitor and mechanical gauge.The monitor system could alarm at the pre-specified threshould.Conclusions Level detector and pressure monitor could be safely and effectively applied to clinical work for perfusion safety.