A femtosecond pulse downconverts broadband electromagnetic signals to the hundreds of MHz range, enabling fast measurement and enhancing electromagnetic monitoring efficiency. However, due to the high peak power of optical pulses, the received power of the detector is severely limited, and thus the sensitivity of the system is reduced. To address this problematic issue, this paper presents a multichannel parallel high-saturation optical power-balanced photodetection technique that uses parallel reception to enhance the detector's received power, thereby improving the signal-to-noise ratio (SNR) and system sensitivity. To achieve the synthesis of multiple received signals, a miniature power divider filter, which achieves high amplitude and phase consistency through a slow-wave transmission structure loaded with interdigital capacitors and serpentine inductors, is suitably designed. The operating frequency of this structure ranges from 105 to 210 MHz with an amplitude imbalance of less than 0.01 dB and a phase difference of less than 0.04°. Compared to similar designs, the area is reduced by 92.3%. The designed high-saturation photodetector allows an input saturation light power of 15 dB, and the detection system achieves a sensitivity of better than 4 mV/m. In the frequency range of 3-12 GHz, the sensitivity remains above 0.3 mV/m, representing a 14 dB improvement over commercial detectors.