The present invention provides an encryption determining method. The method includes the following steps: receiving a side channel signal; generating a filtered side channel signal by filtering noise within the side channel signal; generating a phasor signal by utilizing a filter to covert the filtered side channel signal; locating the encrypted segment by calculating a periodicity of the phasor signal utilizing a standard deviation window; extracting at least one encrypted characteristic from the encrypted segment; and generating an encryption analytic result by recognizing the at least one encrypted characteristic according to a characteristic recognition model; wherein the encryption analytic result includes a position of the encrypted segment within the side channel signal, and an encryption type corresponding to the side channel signal. The present invention is able to automatically and efficiently locate the encryption segment and analyze the encryption type corresponding to the side channel signal.