Magnetic beads are widely used in biological experiments, such as immunoassay and cell separation. The detection of magnetic concentration is profound to the evaluation of experimental results. Therefore a magnetic beads concentration measuring system with dual optical path based on laser transmission method was presented. The concentration of magnetic beads was determined by measuring the ratio of the intensity of incident laser to the intensity of the transmitted laser. A series of magnetic bead suspensions with various concentrations were tested. For each concentration, the measurement was performed for many times and a total of 8,000 sets of data were collected. By fitting the measured data, the relational function between beads concentration and output data can be obtained. The relation between the concentration of magnetic beads and output data was well fitted exponentially. The fitting residual sum of squares is 0.000 088 89 and the determination coefficient is 0.997 1, which illustrate good fitting ability of the model. The experimental results show that the relative measurement error is less than 2.5% and the output data fluctuates within 2.5%, which indicate that the beads concentration measurement system based on laser transmission has high measurement accuracy and good repeatability.