[en] Second harmonic generation(SHG)is widely used process in nonlinear optical spectroscopy. If the input light field is Gaussian beam, the dynamics of SHG is determined by the power of the input beam, the spatial shape of the input beam, its confocal parameter, the Poynting vector walk off length, the phase matching condition and so on. Our goal in this study is to investigate the spatial filtering process by aperture, the spatial shape of the focused input fundamental beam and its confocal parameter. Particularly we study the power characteristic of cw SHG for the periodically poled lithium niobate