A method of dynamically determining a maximum polyphony number is used in an electronic device, which has S tone generators. First, an operation mode of the electronic device is detected. Next, a residual computing power of the electronic device is obtained according to the operation mode. Then, a maximum polyphony number is determined according to a constant computing power, which is required to synthesize a polyphony, and the residual computing power. Next, the states of T tone generators of the S tone generators are set to be an ON state according to the maximum polyphony number, wherein T is a positive integer and is smaller than or equal to S.