The organic ionic salt of trans-4-［4-(dimethylamino) styryl］-1-methylpyridium p-toluenesulfonate (DAST) is a kind of nonlinear optical crystal. Large and high-quality DAST crystals were grown in a saturated DAST-methanol solution from seed crystals by controlling the temperature. The effects of preparation method, temperature and cooling speed on the growth of DAST crystals were investigated in detail. The light transmittance and thermal stability of prepared DAST crystals were studied. The sample of DAST crystal heated at 265℃ was characterized by X-ray diffraction (XRD) and Fourier transformation infra-Red spectroscopy (FT-IR), and it was proved that the phase transition temperature of DAST crystal is 260℃, and it is feasible to prepare DAST crystal from the melt. At the same time, the process of weight loss of DAST crystal at 37—800℃ was discussed preliminarily.