The phloem acts as the main pathway of long distance transport and allocation of nutrients, and as a conduit of inter-organ communication in vascular plants. As a carrier of the dynamic photosynthetic and post-photosynthetic carbon isotope fractionation signal, δ¹³C value of stem phloem water soluble sugars can be a reliable indicator of whole canopy stomata conductance(Gs) to reflect plant responses to the environmental variables in the plant canopy. Therefore, quantifying the stable carbon isotope fractionation of stem phloem and the extent such fractionation would be impacted by outer environmental variables is the key to understanding plant stem phloem carbon pool discrimination, and interactions between plants and outer environmental parameters. However, nearly all current studies of phloem carbon isotope fractionation of conifer trees had been limited to only several conifers in temperate forests, and there is little information about the impact of abundant availability of water and light, and higher temperature on different conifer tree species in subtropical monsoon ecosystem. Given the importance of exploring plant responses and adaption to outer environmental variables, there was a need for more studies of phloem stable carbon isotope fractionation of additional plant groups in additional ecosystem. The paper measured δ¹³C value of stem phloem water soluble sugars of Pinus massoniana and Cunninghamia lanceolata for 6 consecutive days from April 20 to 25, 2011, and related such values to the daily mean solar radiation(Sr), relative humidity(Rh), water vapor pressure deficit(VPD) and air temperature(Ta) calculated on the sampling day and for 1-7 days before the sampling day, meanwhile combining precipitation(PPT) data for further analysis, in order to reveal the day to day dynamics of stable carbon isotope discrimination of newly assimilated photosynthates in P. massoniana and C. lanceolata stem phloem, and its responses to the studied meteorological factors in subtropics during the spring season. The study showed that the daily mean δ¹³C values of stem phloem water soluble sugars of P. massoniana and C. lanceolata fluctuated between -26.81‰ and -26.49‰, and between -29.26‰ and -27.47‰, with an average of (-26.58±0.12)‰ and (-28.67±0.65)‰ respectively. δ¹³C values of stem phloem water soluble sugars of P. massoniana was significantly related to daily mean Sr, Rh, VPD and Ta, but with a time lag of 4 days. Except Ta, such values of C. lanceolata stem phloem were significantly related to the other three meteorological factors, with a time lag of 3 days. Among environmental variables studied in the paper, Sr had the foremost impact onδ¹³C values of stem phloem water soluble sugars both of two conifer species. Rainfall event on the day of sampling would change the day-to-day dynamic pattern of the δ¹³C value of stem phloem water soluble sugars. The results suggested that δ¹³C values of stem phloem water soluble sugars of P. massoniana and C. lanceolata can be a sensitive record of short-term weather change, thus helping to understand and predict the impacts of the weather change on stable carbon isotope discrimination of plant phloem water soluble sugars.