High speed electrooptic phase modulators play very important roles in the high-speed optical fiber communication system, microwave photonic system, and coherent optical communication system, due to their advantages of bias voltage free and linear modulation. As an intrinsic parameter, the half-wave voltage of an electrooptic phase modulator has been characterized by using an electrical spectrum method and an optical spectrum method in the last two decades. The optical spectrum method is generally limited by the line-width of the laser source and the resolution of the available optical spectrum analyzer, while the electrical spectrum method requires the conversion from phase modulation to intensity modulation before photodetection, since a phase modulator generates a phase modulated optical signal with constant envelope. The major difficulty in the electrical spectrum method lies in the extra calibration for the responsivity fluctuation in the photodetection. In this paper, a novel self-calibrated measurement of half-wave voltage of electrooptic phase modulators is carried out based on the optical heterodyning between the two-tone phase modulated sidebands and the frequency-shifted carrier. The method achieves a self-calibration measurement, and avoids the effect of the responsivity fluctuation in the photodetection by setting a specific frequency relationship between the two-tone microwave signals. Moreover, it extends the measuring frequency range to the double bandwidth of photodetection and spectrum analysis. Compared with the optical spectrum method, the proposed method achieves very high frequency resolution measurement, and simultaneously avoids the line-width influence of laser source by use of two-tone heterodyning. Compared with the traditional electrical spectrum method, our method works under no small-signal assumption nor photodetection calibration, and eliminates the limits of electrical driving amplitude and operating wavelength. Moreover, it decreases by at least half bandwidth requirement for the photodetector and spectrum analyzer. Our experimental demonstration shows that the measured half-wave voltages of the electrooptic phase modulator obtained by our method agree well with the data measured by the optical spectrum method, and the two-tone heterodyning method greatly improves the measurement range and frequency resolution. The proposed measurement method provides a very simple analysis method for the microwave characterization of high-speed electrooptic phase modulators, which is also a reference for other optoelectronic devices.