Accurate measurement of the high performance photomodulator is increasing in need. A 40 GHz high-speed measurement system has been set up to accurately measure the frequency response of a 35 GHz photomodulator. The main instrument of the system is Agilent′s vector network analyzer (VNA). An error eliminate model has been derived to obtain the intrinsic response of the modulator, and the result fits well to the manufacturer report. In addition, the eye diagram of the modulator is measured when the modulator is adjusted to the best work status according to the pre-measured modulator curve. And voltage of pi phase shift is measured using power sweep method. All other important parameters are also measured and analyzed.