Abstract: The discontinuities of physical fields (such as density, viscosity and so on) exist in the different phase interface of multiphase flow problems, and the numerical simulation method using the traditional SPH model is liable to cause spurious~oscillations in the pressure and velocity field at the interface, which is a big problem in the application of multiphase flows. An improved SPH model based on Riemann solution on dealing with the abrupt physical quantities of multiphase flows with large density radios is presented. Using the advantage of Riemann solution in dealing with the contact discontinuity problems, we introduce it into the SPH multiphase flow model. For the sake of accurately calculating the physical viscosity of multiphase fluid and decreasing the Riemann dissipation, the SPH momentum equation of the Riemann solution form is improved. In the new model, we combine the Adami fixed particle wall-boundary with the one-sided Riemann problem to impose solid boundary of the SPH multiphase flow, and consider the influence of the surface tension on the small-scale interface. The new model without adding any artificial viscosity, artificial dissipation and non-physical treatment technology can simulate the real physical viscousity and the physical evolution process of multiphase flow problems. In order to verify the ability of the improved model in dealing with the multiphase flow problems with the discontinuous interface and the convergence of the model particle spacing, firstly the squared droplet oscillating problem is simulated under different discrete particle spacing. Afterwards, the multiphase flow problems of the Rayleigh-Taylor instability, the single bubble buoyancy and the double bubble buoyancy are simulated. The interface is clearly capture and the results are good in agreement with the literature, which proved that the improved multiphase flow SPH model can stably and effectively deal with the problems of the multiphase flow large with density ratio and large viscosity ratio.