Water is vital for plant growth and development. However, profuse water molecule present in leaves can increase the possibility of frost or cold injury in winter at temperate region. During frost nights, reduced irrigation or water stress could be effective to reduce frost or cold injury specially in tea plant. THz radiations unlock many interesting possibilities, ranging from fundamental science through to applications in communications, noninvasive imaging, and other areas of nondestructive analysis. Terahertz time-domain spectroscopy (THz-TDS) was used to analyze water absorption coefficient, transmission coefficient, refractive index, and other dynamics in fresh tea leaves (Camellia sinensis). We investigated potted tea leaves of two Chinese tea cultivars named Longjing and Maolu which are common and widely consumed in China as green tea. Irrigation of full (100%), 75%, 50% and control (10%) were applied to potted tea plants of each variety separately. The goal of our research was to find out how tea leaves responses with different irrigation and to assess water stress dynamics to cold injury. We considered fresh leaves with ambient temperature (3 °C) and frequency range of 0.1 to 1.6 THz for all the graphical analyses as within this range water dynamics were shown precisely. Among all the treatments, the highest absorbance trend was showed by Longjing-full and the lowest one by Longjing-control. However, Longjing-50% also showed similar lowest absorbance trend. Maolu (control and 50%) showed significantly higher absorption trend then Longjing while Maolu (75% and 100%) showed slightly lower trend. All treatments of Maolu other than full were shown higher index of refractive than Longjing. Highest transmission coefficient was found in Longjing-50% which represent lowest water presents in the leaf followed by Longjing-control. We concluded that Longjing tea variety with control or minimum irrigation can show more resistant to cold injury than Maolu and hence recommended very low irrigation apply during winter at very low temperature to reduce loss from frost or cold injury. Further understanding and investigation of plant-water relations and water-stress tolerance at the scale of physiology and molecular biology can significantly improve plant productivity and environmental quality.