This paper presented a silica-based 1×8 optical power splitter at 0.85μm designed with Multi Mode Interference (MMI). The waveguide material is Si-based SiO2 doped with Ge and deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition) method. The refractive index of glass substrate is 1.458, and the index difference is 0.75%. The input and output waveguides are optimized considering the characteristics of VCSEL (Vertical Cavity Surface Emitting Lasers). The core pitch on this chip is chosen as 250μm to take the fiber connections into account, and the typical cladding diameter of optical fibers as 125µm. The critical parameters in the fabrication of the MMI power splitter are the multimode section width and length. In general the key performance specifications of the optical power splitter are insertion loss and uniformity. We use a Finite Difference Beam Propagation Method (FDBPM) to simulate accurately the evolution of the fundamental mode power of the input guide incoming to the device, and analyze the relationships of various parameters in detail. The bend output waveguides are sin-bend styles. Compared with the commercial optical power splitter, the simulation results accord with the requirement of our design, containing compact size, low loss, stable splitting ratio, low crosstalk, large optical bandwidth, and good fabrication tolerances. The insertion loss and the uniformity of pilot study are 9.07dB and 0.02dB respectively.