A downstream power calibration method comprises the following procedures. First, select base sampling frequencies and sampling power levels, thereby generating base sampling signals. Next, input base sampling signals into a modem such that obtaining each sum of the feedback signals responding to the tuner and the IF amplifier of the modem by the modem chip corresponding each said base sampling signal, thereby setting up a sampling data table. Third, repeat the procedure mention above for a plurality of modems to get a plurality of sampling data tables respectively, and then obtain a mean data table. Fourth, select spot frequencies and spot power levels from the base sampling frequencies and the base sampling power levels to generate spot sampling signals. Thereafter, input the spot sampling signals into a cable modem and set up a skeleton data table. Finally, expand skeleton data table and obtain an individual default data table for the modem, thereby providing the basis of the calculation of power levels when the cable modem is receiving downstream signals.