The flat-bottom potential is modified according to some physical considerations for the infrared and ultraviolet asymptotic behaviour of the effective quark - gluon coupling. The propagator functions and mass function for the quark, and the Bethe - Salpeter wavefunctions, mass, electromagnetic form factor, radius and decay constant for the pion are calculated numerically in the framework of the Schwinger - Dyson equation in the rainbow approximation and the Bethe - Salpeter equation in the ladder approximation with the modified flat-bottom potential. All results agree with other theories and experimental results, respectively.