Abstract: Carbon isotopic composition (δ13C), which varies during the photosynthetic activity, can synthetically reflect both stomatal conductance and fixation of CO2. As physiological index of species in environment, δ13C can be used to study the relationship between plant and ecological environment. In this work, we mainly discussed the variations of carbon isotopic composition among distinct environmental factors. Furthermore, we talked about the influencing degree and mechanism. Easy to know that precipitation, temperature, irradiance, salinity of soil, CO2 concentration in atmosphere, etc influence, in different degree, the stomatal conductance and fixation of CO2, responding to change the δ13C value of plants. In these factors, precipitation is the most evident factor to the δ13C values, however, temperature is confound to reflect the δ13C values of plants. Atmospheric CO2 commonly influences the intercellular and ambient CO2 pressure of leaf, resulting to various δ13C values. The technique of carbon isotope can be used to study the ecology of plant, for δ13C values can not only reflect water use in the course of photosynthesis, but also the relationship between carbon isotopic composition and water use efficiency is corrective.