A new phenomenological model of self-organized second-harmonic generation is proposed describing the photoinduced χ ( 2 ) grating formation as a cumulative third-order nonlinearity via a complex χ ( 2 ) encoding function. The theoretical results attained are confronted with recent experimental measurements by Lambelet and Feinberg [Optics Lett. 21 (1996) 925] and the χ ( 2 ) encoding function is constructed for this special case. It is also shown that the efficiency of second-harmonic-generation preparation process is closely related with the mutual phase shift between the second-harmonic radiation generated and the second-harmonic seed, which is dependent upon the light intensities of shaping radiation and varies in the course of χ ( 2 ) writing process. The self-organized parametric down conversion with strong second-harmonic seeding being above the saturation value is proposed for an effective χ ( 2 ) grating formation.