Vertical Cavity Surface Emitting Lasers (VCSELs) have important applications in short range optical interconnects due to the low cost, low threshold, low power consumption and high speed. To develop high speed VCSEL, it is essential to have an in-depth analysis of the physical limits of bandwidth. We focus on the relationship between epitaxial layer design and parasitism. In this work, a high speed six-layer oxide-confined 850 nm VCSEL with modulation bandwidth up to 23.8 GHz was achieved, and parasitic parameters were extracted by fitting the measured S11 data. The results can provide design guidance for the high-speed VCSELs.