Vector network analyzers (VNAs) have become one of the indispensable tools in various fields, such as medicine, material, geology, communication, and etc, due to the capacity of measuring and analyzing the response of the object under test. Conventional VNAs, commonly based on mixing architecture, have to make compromises among accuracy, dynamic range, and bandwidth. In this paper, we propose a wideband photonic vector network analyzer (PVNA) based on wideband direct photonic digitizing. Ultrastable optical trains directly undersample the response signals from objects under test, followed by electrooptic conversion and quantization, obviating the intricate downconversion procedures in traditional VNAs. Adopting existing commercial devices, the proposed PVNA can not only extend the measurement frequency range to 110 GHz or higher but also achieve a high linearity and accuracy performance. To validate the theoretical analysis, we establish an experimental PVNA, realizing a measurable frequency span of up to 40 GHz and a dynamic range of more than 120 dB. A measured scattering parameters of a bandpass filter The experimental result is well consistent with that of a commercial network analyzer.