Nb-doped vanadium phosphorus oxide catalysts were prepared and characterized using a variety of methods, including XRD, FT-IR, XPS, TPD and BET. The catalysts were examined for their ability to promote the aldol condensation of acetic acid with formaldehyde to acrylic acid. Catalyst composition and the conditions used to prepare the catalysts were investigated for their effects on catalytic activity. The addition of Nb changes the properties and catalytic properties of the catalyst to some extent, especially the surface acidity of catalyst and the average valence of surface V atoms, which have the greatest influence on the catalyst. When Nb/V = 0.06, the catalyst surface has the greatest acidity, the valence of surface V atoms is the lowest, and the selectivity (83.4%) and yield of acrylic acid (18.1%) are maximized.