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Understanding the seasonal variation of source-water utilization by trees is important to implement effective methods for forest water management. Here, we studied water-source variations of Larix principis-rupprechtii throughout the entire growing season in an alpine habitat, Luya Mountain (North-Central China). Approximately 2000 sample measurements of precipitation, snow water, soil water, and tree xylem water were collected at weekly/biweekly intervals from September 2013 to October 2014, and water isotopes (delta D and delta O-18) were analyzed. The contribution of soil water in different depths to trees was estimated by using a Bayesian mixing model. We simultaneously monitored leaf phenology and cambial activity throughout the growing season. The latter was observed by the micro-core method based on the characteristics of xylem cells, to identify the specific dates of onset and cessation of the tree’s growth. Our analysis showed several interesting results. First, before needle unfolding (May 16), xylem water loss occurred even in cold-moist alpine conditions. Second, around late May/early June, winter snowmelt water seemed to play an important role for growth onset. Finally, during the growing season (June to August), larch depended on waters from deep soil (below 10 cm), which could be explained by their deep root system, and the maximum soil water content being at a soil depth of 10-20 cm. The mixing water, in addition to summer rainfall, was predominantly recharged by previous-year autumn/winter precipitation (rainfall/snowmelt). This study clearly demonstrates the variation of source-water in different phases, and it emphasizes the contribution of previous-year precipitation for tree growth. Therefore, it is essential to distinguish the timing of tree leaf phenology and growing season when exploring the inter- and intra-seasonal changes in water-source use.