In this paper the well-known solar active region McMath 12094 of Oct. 1972 is taken-as an example to investigate the relation between the magnetic field twist and the flare occurrence rate in a solar active region. Adopting a model of constant a force-free magnetic field for the region and taking the observed photospheric longitudinal magnetic field as the boundary condition, the values of force-free factor a, which represent the average level of magnetic twist in the region, are calculated for the four days when the region was near the center of solar disk (28-31 Oct.). The nusing these a values, the three dimensional configurations of the lines of magnetic force are extrapolated. Based on these data, the evolutionary characteristics of the magnetic field as well as the relation between the magnetic field twist and the flare occurrence rate are discussed. Finally, the electric current in the preceding sunspot Aof the region the electric current density and the stored magnetic energy caused by the on of the sunspot Aare estimated under the approximative model of an unipolar magnetic field. The main conclusions obtained are as follows.1) The magnetic field twist in McMath 12094 was rather strong since 28 Oct. and reached a maximum on30
Oct.. After 31 Oct. it became weaker. The principle source of the magnetic twist was the counterclockwise rotation of the preceding sunspot A, which was caused by MHDin the photosphere.(2) The values of the force-free factor a which represent the average level of the magnetic twist varied in parallel with the flare occurrence rate, indicating that the latter is proportional to the magnetic field twist in the region.(3) The electric currents through the umbra of the large preceding sunspot Aare 4.3-6.6×1012
A. The stored free energies caused by the magnetic twist are 0.44-1.11×1032
erg. The densities of electric current in the region rise up to 0.96-1.47×10-2
. Probably these high densities are responsible for making the active region with high occurrence rate of flares.