Spatial beam method is explored to eliminate the backside reflection in ellipsometric measurement. The main concept of this program is to focus the beam on the of sample surface, diverge on the bottom, and then the back reflection light will not reach the detector. The quartz glass and niobium oxide monolayer film are chosen as the experimental sample. Quartz glass samples are divided into 3 groups, which are measured directly, after backside grinding, and after the spatial splitting, respectively. For the untreated sample testing, the refractive index values at three incident angles are obviously different. In backside grinding case, the refractive index of the quartz glass appears consistent for three incident angles. After the spatial splitting, the refractive index of the quartz glass appears the same. The spatial splitting method is used for ellipsometric test of anti-niobium oxide monolayer, and good fitting of the experimental data is got.