Blade-shaped field cathodes have a significantly larger emission area compared to single point emitters, which makes it possible to increase the total current in the field emission system. This paper presents the calculation of the potential distribution in a diode emission system with a blade-shaped field cathode on a flat substrate. Anode is a plane parallel to the substrate. To calculate the two-dimensional electrostatic potential in the entire area of the system, the variable separation method in Cartesian coordinates is used. The influence of the field cathode on the field distribution is replaced by the influence of a finite number of charged filaments with linear charge densities so that the surface of the real cathode coincides with zero equipotential. The graphs of the electrostatic potential distribution and the equipotential lines in the entire region of the investigated diode system are presented.