Abstract: Abstract: The planting area of pitaya (Hylocereus polyrhizus) is expanding continuously in China in recent years, due mainly to the high economic and ecological value of this tropical fruit . However, drought has become one of the key factors restricting the growth and yield of H. polyrhizus in arid regions. This study aims to explore the characteristics of water consumption subjected to transpiration, and the influencing factors of H. polyrhizus. A Dynamax sap flow measuring system was employed to measure and monitor the sap flow velocity in seven consecutive days per month from September 2019 to August 2020. The planting field was located in the digital agricultural experimental base of Guizhou Academy of Agriculture Sciences in western China. Meanwhile, the related environmental data was simultaneously measured. The correlation of sap flow features with various factors was analyzed to quantify the water demand threshold of H. polyrhizusand water-saving irrigation. The results showed that the single-day sap flow of H. polyrhizus was dominated by a single peak curve. There was an increasing trend of sap flow after sunset, whereas, the sap flow rate varied gently from 00:00 to sunrise. Diurnal sap flow accounted for 49.60%-71.51% of the whole day, whereas, the night sap flow was relatively low. The total sap flow of H. polyrhizus sequenced as spring>winter >autumn >summer. The new shoots emerged sharply in spring, where the peak value of sap flow and average daily flow rate reached the maximum, 2.68 and 3.08 times those in the slow-growing period, while 1.65 and 1.52 times those in the flowering and fruiting period, indicating a leading role of the growth period. However, the proportion of nocturnal sap flow in spring (the average sap flow ratio was 31.05%) was significantly lower than that in other seasons (41.01%-43.65%). The proportion of sap flow velocity at night was ranked in order: summer> winter> autumn > spring. There was a significant correlation between the total night and day sap flow, where the correlation coefficient was 0.917 2. The peaks and valleys of sap flow were scattered with time. Specifically, the peaks appeared more frequently during 10:00-13:00, whereas, the valleys were more concentrated in 17:00-20:00. The sap flow rate had a very significant positive correlation with the total solar radiation (TSR), photosynthetically active radiation (PAR), whereas, extremely negatively correlated with the soil temperature (Ts), soil moisture (Ms), relative humidity (RH), and vapor pressure deficit (VPD). Additionally, the sap flow rate was negatively correlated with the TSR and PAR under a high light intensity (TSR>550 W/m2). Furthermore, the responses of sap flow velocity to each environmental factor were ranked in order: TSR >PAR >Ms>RH >Ts>VPD >Ta. There was a significant positive correlation between the total sap flow during the day and night, but the sap flow rate responded differently to temperature and relative air humidity. During the day, Ta had a weaker effect on the sap flow, while at night a significant negative effect. The RH had a negative effect on the liquid flow during the day, whereas, a positive effect at night. Consequently, there were some effects of seasonal factors on the sap flow of H. polyrhizus at night. Adequate irrigation was therefore very important in the spring for the water and fertilizer management of H. polyrhizus planted in the greenhouses. The finding can provide a technical guideline for further research on the water-fertilizer integrated irrigation system of H. polyrhizus.