The plasma is produced during the monocrystalline silicon material is ablated by femtosecond laser, whose spot image can obtained by the collection of the radiation signals of plasma with CCD camera. Considering the characteristics that the contrast between the edge and background areas of plasma spot is not obvious and the signal to noise ratio is low, the spot image is enhanced and the brightness characteristics of this spot image are further analyzed. Firstly, the original spot images are filtered and then enhanced by the principal component analysis method. The obtained first principal component image is pseudo-color processed in order to analyze its energy distribution, and the core region of the spot is segmented according to its energy distribution information. Secondly, the centroid and long axis of the original image as well as the centroid, long and short axes of the core ablation zone are calculated, and the geometrical features of the image are analyzed. Based on these, the laser ablation direction is determined. Thirdly, the Niblack segmentation method is used to divide the gray image of the original image, and the luminance characteristics of the spot image are obtained based on the luminance information of the segmented binary image mask. Finally, the relationship of brightness characteristics of spot with laser ablation processing direction and laser power is analyzed. The correlation coefficient between brightness characteristics and processing direction is less than 0.09, and the correlation coefficient between brightness characteristics and laser power is more than 0.5. The results show that the luminance characteristics of spot image are not affected by the processing direction and the spot light intensity is stable, which can be used for the identification of laser ablation power.