Abstract: With the application of stable isotope mass spectrometry in soil nitrogen cycle research, nitrogen stable isotope analysis puts higher requirements for the sample pretreatment methods. At the same time, a suitable pretreatment process is also the guarantee of the analysis result accuracy. A automatic pretreatment auxiliary device for nitrogen stable isotope analysis of soil nitrate is designed and developed in this paper. The auto-injection system based on photoelectric sensing device is integrated with the liquid injection and vacuum-gas replacement system to achieve automatic process instead of manual operation, ie., liquid reagent injection, vacuum extraction of reaction system, helium gas displacement and other artificial processes. The results of system test showed that the device has a good vacuum, gas injection and liquid repeatability. The absolute error of the system vacuum degree, gas injection volume and the liquid injection system of the device can be controlled within 0.5 Pa, 0.2 mL, and 0.1 mL respectively, with CV< 1%. The accuracy and precision of the entire system were also tested by using standard reference samples. All test samples obtained good accuracy and precision of 0.1%~0.6% with CV<1%. The developed device combined with the trace gas preconcentration device-stable isotope mass spectrometry instrument (PreCon-IRMS) can establish a convenient and rapid method for the nitrogen stable isotope determination of nitrate at low sample volume, which can provide a key technical support for soil nitrogen cycle process research.