One of the most promising SF6 replacement gases is the perfluoroisobutyronitrile (C4F7N)/carbon dioxide (CO2) gas mixture. The experimental platform for C4F7N/CO2 gas mixture in an extremely inhomogeneous field discharge is built, and a multi-source signal synchronization acquisition device is established to obtain optical and current pulses under different applied voltages. The discharges show a “hump” shape with the increase of voltage. The generation of optical and current pulses is synchronized with complementary phase distributions, i.e., the optical pulses are generated in the negative semiaxis and become stronger in the positive semiaxis; the current pulses are generated in the positive semiaxis and become stronger in the negative semiaxis. The Gaussian noise in the optical and current pulses is removed using the three-sigma principle, and different semiaxis root mean square (RMS) and pulse counts are calculated. At the early discharge stage, optical pulses at negative and current pulses at positive semiaxis have larger RMS and pulse counts. At the late discharge stage, the above phenomena are opposite. This process is related to the polarity effect of the extremely inhomogeneous field. This study provides a reference for detecting extremely inhomogeneous field discharges in C4F7N/CO2 gas-insulated electrical equipment, promoting the application of combined optical-electrical detection techniques.