The paper researched the isothermal time and temperature affects the solid particles evolution of semisolid AZ91D alloy, and the alloy be produced by the method of strain induced melt activation (SIMA) ,and the method of compression ratio is 40%. The research demonstrated that the solid fraction decreased and solid particle size increased when increasing temperature and time. The isothermal coarsening of solid particles obeys Ostwald ripening which depending on the curvature of the interface, thus creating concentration gradients and the diffusion transport of material. As a result, the large particles coarsening and little particles minish even dissolved and creating change of distribution frequency of solid particle size. The fitting curves reveal that the experimental data are well fitted to the coarsening equation. The quantitative analysis of solid fraction and solid particle size demonstrated that the appropriate isothermal holding temperature is 550°C -590°C and corresponding isothermal holding time is (45-60min)-(6-20min) in 40% compression ratio.