The spreading process of an isothermal droplet impinging on flat substrate surface in plasma spraying is studied numerically in 2D cylindrical coordinate systems by using 'Marker-And-cell (MAC) Technique. The changes and distributions of the transient contact pressures upon substrate surface at flattening are calculated under different droplet conditions with different impacting velocities and densities. The simulated results show that the transient contact pressure is initially high and concentrates at a small contacting area, it then spreads and drops quickly while droplet flattens. The maximum pressure is located at the front of the droplet at early stage of deformation, which pushes the fluid moving quickly along substrate surface and results in lateral flow. The contact pressure is mainly related to the droplet density and impact velocity. The peak pressure reduces consistently along the substrate surface so that the splashing at the periphery of flattening droplet may occur to form a reduced disk like splat because of the falling of contact pressure in this region and the escaping of the evaporated gas from the droplet / substrate interface.