The crystalline Si films are prepared by pulsed laser deposition (PLD) in 10 Pa helium gas at room temperature. In experiments, substrates are located under ablated spot and paralleled to the axis of plume, at the same time, a vertical extra helium gas flow apart different distance from target surface is introduced above plume. The results of scaning electron microscope (SEM), Raman scattering and X-ray diffraction (XRD) indicate that nano-crystal grains formed in one area apart from target, the sizes first increase and then decrease with the addition distance from target surface. Size and position distribution of grains on substrates are analyzed. Combing with hydrodynamics model, nucleation division model, thermokinetic equation and rule of flat parabolic motion-like after formation of grains, nulcleation region width of 56.2 mm is calculated through numerical calculations.