In order to achieve precision detection of the weak,divergent and fast modulated optical signal in atomic magnetometer systems and overcome the disadvantages of traditional balanced detectors with small light-receiving areas,small gains,nonadjustable center distances between eachpair of photodiodes and incompatible with free space optical signal detection,anapproachbased on Kirchhoff's law of balanced differential amplification and a fine tuning methodbased on parallel tracks pads are adopted. Ahigh-gain balanced photodetectorisdesigned and produced,and the working principle and structure composition are introduced. First,for the features of large spot divergence and high modulation rate,a high-speed photodiode with a 10 mm×10 mm large light-receiving area isdetermined;then a pair of parallel track padswhich enabledouble-diode center spaceto be continuously adjusted from 20 mm to 60 mm isdesigned,which enhancesthe compatibilitywith a variety of optical systems;Finally,a two-stage amplifier circuit is designed to achieve the high gain characteristic. Experimental results show that the detector achieves a -3 dB bandwidth of 800 kHz,a signal transimpedance gain of 0.91 MΩ,and a SNR of 38.5 dB@ 70 kHz. Therefore,this balanced photodetector fairly meets the needs of optical signal detection of atomic magnetometer systems.