An optical stress sensor is proposed and demonstrated in experiment based on single bismuth germanate (BGO) crystal and electrooptic compensation. The sensing principle is mainly based on the mutual compensation property between photoelastic birefringence and electrooptic birefringence existing in the BGO crystal. According to the index ellipsoid analysis method, when external stress and electric field are simultaneously applied to the (111) crystalline surface, photoelastic birefringence of the BGO crystal can be compensated by its electrooptic birefringence, thus it is possible to perform the closed-loop measurement of external stress by the method of electrooptic compensation. The proposed optical stress-sensing unit is only composed of two prism polarizers and single parallelogramic BGO crystal. Additional quarter waveplate is not needed for the proposed stress sensor since the optical phase bias of 0.5π is produced by two times of total inner reflection of light wave in the BGO crystal itself. Compression stress is measured in the range of 30 kPa and experimental data demonstrated a good linear relationship between compensating voltage and measurand stress. The compensating voltage is about 4.26 V corresponding to the applied stress of 1 kPa.