Abstract: In BEPCⅡ, button BPM and stripline BPM cannot reach sufficient precise resolution for beam transverse displacement. This project aims at the design of cavity BPM for BEPCⅡ linac. Position cavity in cavity beam position monitor (CBPM) is an re-entrant resonator with four rectangular waveguides. TM₁₁₀ mode frequency is chosen in S band, and the radius of beam pipe is 23 mm. TM₀₁₀ mode in reference cavity is almost as same as TM₁₁₀ mode in position cavity. According to the results of offline test, characterized parameters of CBPM coincided with computer simulated data. The frquency of TM₁₁₀ mode are 2502 MHz in horizontal direction and 2503 MHz in vertical direction. Cross-talk isolation for position cavity is better than −44.7 dB. Ratio front circuits included filtration, amplification and down-convertion when signals extracted from CBPM. Offline caliberation test results showed that CBPM has excellent measurement value of linearity area, which is over 10 mm. After frequency spectrum analysis and linear fitting, the position resolution of CBPM is 2.87 μm in horizontal direction and 2.16 μm in vertical direction.