A time lag between sap flux density (Js) and meteorological factors has been widely reported, but the controlling factors of the time lag are poorly understood. To interpret the time lag phenomenon systematically, thermal dissipation probes were placed into each of eight trees to measure the Js of Larix principis-rupprechtii Mayr. in the Liupan Mountains in Northwest China. Meteorological factors, including vapor pressure deficit (VPD), solar radiation (Rs) and air temperature (Ta), were synchronously measured with Js, and the dislocation contrast method was used to analyze the time lag between Js and the meteorological factors. The analysis indicated the following for the whole experimental period. (1) The time lag between Js and VPD (TLV) and the time lag between Js and Rs (TLR) both exhibited different patterns under different weather conditions, and Js could precede Rs on dry days. (2) Both TLV and TLR varied with the day of the year (DOY) throughout the experimental period; namely, both exhibited a decreasing tendency in September. (3) Reference crop evapotranspiration (ETref) had a greater influence on the time lag than the other meteorological factors and directly controlled the length and direction of TLV and TLR; relative extractable water (REW) modified the relationship between ETref and time lag. (4) The regression analysis results showed differences between the time lags and the environmental factors (ETref and REW) within different ranges of REW. Namely, TLR was better determined by ETref and REW when REW < 0.38, while TLV was better correlated with ETref and REW in the absence of soil water limitations (REW > 0.38). This project provided an important opportunity to advance the understanding of the interaction between plant transpiration and meteorological factors in a changing climate.