In the magnetic fields built by neodymium–iron–boron (NdFeB) permanent magnets, the corresponding relationship among the concentration of sodium chloride solution, glucose solution and their magnetic rotation angles was researched in the temperature range 20–24°C. A helium–neon (He–Ne) laser with a wavelength of 632.8 nm was used in the experiment as light source. A new device is designed for assembling and adjusting the NdFeB permanent magnets. The permanent magnets have no thermal effect compared to an energized solenoid. The corresponding relationship between the concentration and rotation angle is calibrated by standard concentration gradient solutions. Then the concentration can be detected by using this relationship. The rotation angle of the solution is proportional to the concentration and increased with an increase of the magnetic induction intensity. In the magnetic cavity with Φ12 mm dimension, the rotation angles of sodium chloride solutions in the range 0–30 g/100 ml vary from 2.66° to 3.73° and their relative errors range from −1.39% to 1.34%. The rotation angles of glucose solutions in the range 0–50 g/100 ml vary from 2.66° to 26. 44° and their relative errors range from −0.55% to 1.00%. The uncertainty of this system is 0. 05°.