Abstract: It's an essential step for the commissioning of electron-positron collider to realize the absolute collision of electron and positron beams at the interaction point. The relative transverse offset between electron and positron beams causes the beam-beam deflection, which can be utilized effectively to detect and control the offset of two beams at the interaction point. Observation and measurement of the beam-beam deflection in vertical plane have been carried out in Beijing electron positron collidor (BEPC) successfully with a digital high-sensitive oscillograph and a pair of electrostatic separators. The study results not only prove the feasibility of this adjustment method but also provide valuable experience for the commissioning of BEPCⅡ.