Abstract: [Objective] Stem flow is a main index to study on transpiration law, therefore accurate and reliable stem flow measuring device is essential to support the study. [Method] This paper designed a FLGS - TDP sensor based on thermal diffusion theory and designed a data acquisition system based on MCGS. [Result] The data acquisition system has been calibrated through simulation experiment. And the experimental results show that the deviation between the measured stem flow velocity and real velocity is less than 0.01mm/s which means the stem flow measuring system has higher precision and higher stability. To take poplar as example to make a experiment. The results show that: after sunrise, with the temperatures rising, stem flow velocity begins rising sluggishly. It reaches its peak at about 12:00 to 15:00 and maintains a higher velocity during this time. Since then, stem flow velocity declines slowly. Stem flow velocity begins to decline rapidly after sunset at about 19:00. Hereafter it maintains a lower velocity until the next day. [Conclusion] All above shows this system can accurately indicate the diurnal variation of stem flow with good stability and can also afford the device support to the study on plant water consumption and transpiration.