Larix gmelinii forest plays an important role in the structure and function of forest ecosystem in Northeast China. In the context of climate change, the increase of temperature and the decrease of light in Northeast China will affect the change of Larix gmelinii forest, especially its phenology, a sensitive index of climate change. Understanding the response of phenology to climate change is helpful to reveal the change trend of Larix gmelinii. However, the response mechanism of Larix gmelini phenology to temperature, photoperiod and its synergistic effect is still unclear. According to the fact of increasing temperature and decreasing sunshine duration in Northeast China, a large-scale artificial climate chamber simulation experiment was conducted from April to November 2019 to understand the response of main phenological stages of Larix gmelinii seedlings with 3-year old to the changes of temperature and sunshine duration. The results indicate that: (1) Different degrees of warming had no significant effect on the growth cycle of Larix gmelinii (from the first stage of leaf unfolding to the complete discoloration stage), but significantly shortened the peak growth period (from the peak stage of leaf unfolding to the leaf discoloration in general). The effect of 2.0 ℃ warming on the growth peak was greater than that of 1.5 ℃ warming. (2) The growth cycle of Larix gmelinii significantly increased (9.25 days) with long sunshine duration, but the effect of short sunshine duration on the growth cycle of Larix gmelinii was not significant, and both long and short sunshine duration shortened the peak growth period of Larix gmelinii, and had the same effect on the peak growth period of Larix gmelinii. (3) The interaction between sunshine duration and increasing temperature shortened the peak growth period of Larix gmelinii, and the interaction between 2.0 ℃ warming and long sunshine duration had more significant to shorten the peak period of growth (34.67 days). (4) Compared with the initial growth stage (the first stage of leaf unfolding) and the final growth stage (complete discoloration stage), the peak growth stage of Larix gmelinii was more sensitive to temperature. The temperature sensitivity of phenology of Larix gmelinii decreased with the increase of temperature except the leaf discoloration in general. Under the conditions of different increasing temperatures, the decrease of sunshine duration decreased the temperature sensitivity at the leaf discoloration onset, however, when the temperature increased by 1.5 ℃, the decrease of sunshine duration decreased the temperature sensitivity at the peak stage of leaf unfolding and increased the temperature sensitivity at the leaf discoloration in general, while it was the opposite under 2.0 ℃ warming. This indicates that climate change in Northeast China is not conducive to the growth of Larix gmelinii, and that 2.0 ℃ warming is more unfavorable than 1.5 ℃ warming. The results can provide a basis for understanding the changes of ecosystem structure and function in Northeast China.