To clarify the responses of tomato stem sap flow to soil water availability and microclimate in an extremely arid region, three irrigation treatments were established in an overwintering solar greenhouse in Kashgar, Xinjiang. The soil at the experimental site is sandy loam: full irrigation (CK, 80–90% field capacity), mild deficit irrigation (T1, 70–80%), and moderate deficit irrigation (T2, 60–70%). Stem sap flow, root-zone soil moisture, and canopy microclimate were continuously monitored to characterize sap flow dynamics, environmental correlations, and time-lag responses across growth stages and weather conditions. Stem sap flow exhibited a clear unimodal diurnal pattern under all treatments. On sunny days, the peak sap flow of CK decreased from 39.62 cm h⁻¹ during the flowering and fruit-setting stage to 20.98 cm h⁻¹ during the mature harvesting stage. Sap flow dynamics under T1 were generally similar to those under CK, whereas T2 showed a marked reduction under high radiation and vapor pressure deficit conditions. Sap flow lagged behind solar shortwave radiation by approximately 30–60 min. Its association with microclimate also varied with growth stage and weather conditions, with solar radiation showing a strong correlation during the flowering and fruit-setting stage and air temperature becoming more closely associated with sap flow during the mature harvesting stage. Compared with CK, T1 reduced irrigation water use by 11.5–30.1% and yield by 10.7–16.5%, while increasing irrigation water use efficiency by 1.2–19.0%. Under the present experimental conditions, maintaining root-zone soil moisture at 70–80% field capacity provided a favorable trade-off among plant water transport, yield, and water saving. Stem sap flow may therefore serve as a supplementary physiological indicator for irrigation management in solar greenhouse tomato production.