Abstract: Due to the fact that the thermal flowmeter with constant temperature difference brings large errors when measuring downhole flow in low flow-rate, the influence of temperature and pressure was studied. According to the relationship between the electric power of the flowmeter heater and the measured flow rate, the influence of temperature on the physical parameters of liquid water was analyzed, and the change curve of the heat transfer power of the flowmeter and flow rate at different temperatures was simulated numerically. An experimental platform with adjustable water temperature and flow rate was built on that basis, and the relationship between the power of the flowmeter and flow rate at 25–40 °C under constant pressure was thereby analyzed, revealing that power of the flowmeter heater increases monotonically with temperature. In terms of theoretical analysis, a 500 m increase in the well depth leads to an error of 2.2 m³/d when the flowmeter is in the depth from 0 to 2 000 m; while the error is 0.6 m³/d when it is in the depth from 2 000 to 4 000 m. The results showed that output power of the thermal flowmeter with constant temperature difference will increase with the increase of well depth (wellbore temperature, pressure change), resulting in measurement errors, which provides a theoretical basis for the correction and effective application of the flowmeter measurement results.