Vector network analyzer is an important measurement instrument in the fields of radio and microwave. In this paper, we design and implement a microwave photonic vector network analyzer (MPVNA) based on optical sampling, which uses ultrastable optical pulse trains, generated by mode-locked laser, to directly sample single tone signals via a Mach-Zehnder modulator, and then processes the sampling data with digital signal process to obtain the scattering parameters of the device under test. The experimental result shows that MPVNA bandwidth is 20 GHz when using an electro-optic modulator with bandwidth of 20 GHz in the system and a larger system bandwidth can be achieved by using an electro-optic modulator with larger bandwidth. The system dynamic range is about 60 dB and the minimum system frequency resolution is 11.92 Hz. Then we use this system to test the scattering (S) parameters of a bandpass filter with a center frequency of 10 GHz and compare the test result with that of a commercial vector network analyzer. The comparison results show that the average amplitude deviation of S₂₁ is 0.1241 dB and the average phase deviation of S₂₁ is 3.6356° in the passband of the bandpass filter, which proves that the test results of the system is well consistent with that of a commercial vector network analyzer.