The theme of his masterpiece Flower of Evil is evil and its development around it. Evil refers not only to evil, but also to depression, pain and disease. Flowers can be understood as goodness and beauty. Baudelaire broke the view of good and evil for thousands of years and observed evil from a unique perspective. Topics involve tramps, prostitutes, prisoners, crimes, etc. At that time, romantics despised the "ugliness and evil" that never entered poetry.
For example, in Melancholy Four, all the images in the poem are ugly: pot cover, black light, damp prison, timid bat, rotten ceiling, bars behind bars, dirty spiders, cobwebs, wandering ghosts, long hearse procession and black flags. These disgusting, ugly and ominous images come one after another and fill the whole poem. They showed "mental turmoil".
Baudelaire regards ugliness as beauty and turns ugliness into beauty, which is of innovative significance in aesthetics. This aesthetic view has become one of the principles followed by modernist literature in the 20th century.