A simplified 180° focusing β ray spectrometer was described. The average radius of the trajectories of the electrons in the spectrometer is 100 mm. The uniform magnetic field of the spectrometer is produced by an electromagnet of single york. With two pole faces as bases, the vaccum chamber has D shape side walls. They are sealed together with rubber gaskets. The radioactive source and the G-M counter can be set along the flat part of the side wall. The magnetic field of the spectrometer is measured by a "zero point magnetometer". Its measuring coil is fixed on a tungsten filament in tension. In order to keep the coil, which is mechanically unbalanced, in a given direction, we must pass a current through the coil, so that the mechanical moment can be counteracted by the moment produced by the interaction between the current in the coil and the magnetic field of the spectrometer. As the mechanical moment is fixed for different magnetic field the current passing through the coil must be varied in order to make the balance.The potentiometric readings of the current can be taken as the measures of the magnetic field. Including the magnetometric measurement, the readings of the spectrometer as a whole have their linearity and precision both at about 0.1%. After a series of rearrangement and improvement, the spectrometer attains its best resolving power for a Hg198 K conversion line with a source of 0.8 mm width. It is 0.7%. For Ba137K conversion line with a source of 2 mm width it attains a resolving power of 0.9%.The essential merit of the spectrometer is its simplicity both in construction and in operation.