Considering the sensitivity of the refractive index to the temperature in PPLN and the thermal expansion character of the LiNbO 3 crystal,the grating period expression and the temperature dependency characterization of quasi phase matched second harmonic generation (QPM SHG) bulk devices fabricated in Z cut LiNbO 3 crystal are theoretically analyzed.The results show that the max conversion efficiency of SHG can be obtaied by tuning the temperature in the PPLN to approach the center temperature.The relations between the temperature tuning bandwidth and the interactive length and the fundamental wavelength are also given.The theoretical results fit well to many author's experimental achievements.