The stream formation in a 1-atm nitrogen gas switch is investigated by the two-dimensional and three-velocity (2D3V) particles through the cell-Monte Carlo collision (PIC-MCC) simulation and theoretical analysis. For simplicity, two parallel plane electrodes of 0.6 mm width are separated by a distance of 1.6 mm. It is found that the analytical solution of the electron density equation can be used to study the evolution of the plasma before the stream breaks down, for the ionization frequency, mean electron energy and electron drift velocity are all constant. After the breakdown of the stream, random collisions destroy the symmetry of the plasma region and cause plasma to branch. As plasma density increases, the electric field inside the plasma region decreases due to the shielding effect. However, charge densities at both ends of the plasma region increase and the density at the anode end is larger than that at the cathode end, for the plasma exponentially grows as electrons move from the cathode toward the anode. This causes the electric field at the end of plasma near the anode to be larger than that near the cathode. It is found that the electrons can achieve their stable mean energy in several picoseconds due to the high transfer frequency (1011-1012 Hz) of the electron energy in the nitrogen plasma. After the breakdown of the stream, the mean electron energy decreases due to the decrease of the electron energies inside the plasma. By increasing the electrode voltage, it is found that the mean electron energy increases, the electron drift velocity increases linearly, and the variation rate of ionization frequency with electric field is in a range between E4 and E5. Therefore, the time taking for breaking down the stream decreases with the increase of the electrode voltage.