This paper presents a physics-based method for simulating the phenomenon of ink diffusion in fluid. Here are two contributions in our method: 1) Our method is physics-based method, which can simulate ink diffusing phenomenon realistically. 2) CUDA acceleration and spatial subdivision optimization is applied to our method, thus our method can obtain a realistic simulation result in real time. This method is a grid-particle combined method: We use two different particle systems to represent fluid and ink, and use a improved smoothed particle hydrodynamics(SPH) method to simulate the behavior of fluid, velocities of ink particles are calculated by neighbor fluid particles. In our method, Compute Unified Device Architecture(CUDA) acceleration technique and the space subdivision optimization are applied to achieve a real-time simulation. The rendering of ink particles uses point sprites and blurring techniques. Compared to other ink simulation methods, our method is more realistic and efficient.