Objective To compare the prefrontal electroencephalogram‑spectrogram before and during mild hypothermic cardiopulmonary bypass (CPB) under propofol anesthesia, and analyze its characteristics. Methods From May 2019 to September 2020, 29 cases of coronary artery bypass grafting surgeries with propofol titration under mild hypothermic CPB were analyzed. This study compared β, α, θ, and δ wave powers between the pre‑CPB state and mild hypothermic CPB state; this study plotted the electroencephalogram‑spectrogram during the CPB procedure; average electroencephalogram‑spectrograms of pre‑CPB state and the mild hypothermic CPB state were reconstructed and compared; this study made a differential spectrum analysis to describe the spectral change between these two states. Results Electroencephalogram‑spectrograms were dynamic changing during CPB. Compared with baseline, mild hypothermia CPB state (at 30 ℃) increased in θ power [2.8 (1.9, 3.9) dB, Z=−4.64, P<0.001], but decreased in β power [−3.4 (−4.1, −2.6) dB, Z=−4.62, P<0.001], α power [−2.9 (−3.9, −2.0) dB, Z=−4.01, P<0.001] and δ power [−1.5 (−1.9, −0.9) dB, Z=−3.53, P<0.001]. The electroencephalogram‑spectrograms intuitively displayed this change， it presented a dynamic decrease of frequency in the range of α‑θ during cooling, and a dynamic increase of frequency in the range of α‑θ during rewarming; the differential spectrum indicated that the state of mild hypothermic CPB had more power in the frequency within 4−8 Hz (P<0.05 in range of 4−8 Hz). Conclusions This study suggested that the electroencephalogram‑spectrogram significantly changed during mild hypothermic CPB, the increase of θ power and the decrease of β, α, and δ powers might be regarded as a reference for judging the state of anesthesia in titrated propofol maintenance.