The inner flow field of biogas plant can be optimized with agitation for the even distribution of fermentation material and the rise of biogas production rate. The inner flow field can be greatly optimized by bottom hydraulic agitation with high dispersed design for outlets. Hydraulic agitation is essentially a solid-liquid two-phase flow process, in which large solid particles can be found at the initial stage and turn to be high-concentration viscous liquid (non-Newtonian fluid) state. A 0.75m³ anaerobic digester was used as a case study. This study was conducted at the base of Computational Fluid Dynamics (CFD) software. The basic pattern was simulated by using the water as the medium and the later pattern of pseudo plastic fluid state was simulated by using the Euler-Euler Model, then the effect of optimal design concepts with bottom inflow and high dispersed outlets could be verified. Analyzing the agitation effects from different inlet velocity, velocity of 0.6 m/s is better than 1 m/s for water medium, meanwhile, velocity of 1m/s is better than 0.6 m/s for straw medium.