A bias-independent inter-modulation method is proposed and demonstrated for measuring low-frequency modulation and bias half-wave voltages of Mach-Zehnder modulators (MZMs). The method consists of simultaneous sinusoidal modulation on the modulation and bias ports of the MZM under test. Sinusoidal-modulated sidebands heterodyne with each other and generate the desired inter-modulation products after photodetection, which allows extracting both the modulation depth and half-wave voltage for the modulation and bias ports of the MZM. Our method is independent of bias voltages of the MZM, which can be canceled out by carefully choosing the sinusoidal-modulation frequencies. Moreover, the proposed method enables the low swing voltage for measuring both the modulation depth and half-wave voltage of MZMs. Experiments indicate that the proposed method features the simple setup and high accuracy for low-frequency response measurement ranging from 1 Hz to 1 MHz.