In this paper, the tungsten oxide photocatalysts with monoclinic/hexagonal phase junction structure (m/h-WO3) were synthesized from in situ growth of monoclinic tungsten oxide on the surface of hexagonal tungsten oxide by hydrothermal method when the sodium tungstate dihydrate was used as the main raw material. The results of XRD, Raman and SEM characterization demonstrate that the composition controll of the m/h-WO3 phase junction structure can be realized by adjusting the concentration of the reaction solution. The experimental results of photocatalytic degradation of Rhodamine B under visible light show that compared with pure phase h-WO3 and m-WO3, m/h-WO3 phase junction structure has better photocatalytic properties. The composition of hexagonal phase and monoclinic phase in m/h-WO3 phase junction is one of the main factors affecting photocatalytic performance. The 5% m/h-WO3 phase junction sample has exhibited remarkable photocatalytic activity, and the degradation rates were 7.6 and 5.0 times than that of h-WO3 and m-WO3 samples, respectively. SPV test results show that m/h-WO3 phase junction structure, especially with a suitable crystalline phase composition, can significantly improve the separation efficiency of photogenerated electrons and photogenerated holes, thus improving the photocatalytic performance of the photocatalysts.