Balanced homodyne detectors have the disadvantages such as small common mode rejection ratio (CMRR) and low response bandwidth, etc. To address the problems, we designed a scheme to test the response characteristics of the photodetector (PIN photodiode) in the detector and selected devices with similar performance for balanced detection to improve the CMRR. Furthermore, the scheme of cascading transimpedance amplification and proportional operational amplifier were proposed to extract the photocurrent signal, which solved the problem of the limitation of the gain bandwidth product of the primary sampling amplifier and improved the response bandwidth of the detector while ensuring the sensitivity. As a result, we achieved 300-MHz balanced homodyne detection and the CMRR went up to 66 dB. We also detected the quantum shot noise in the optical signal. These achievements provide an effective technical scheme for high-speed sensitive detection.