The invention relates to a cloth surface defect online monitoring device. The problems in the prior art are solved. Key points of the technical scheme are as follows: the device is arranged on a clothproduction line, the device comprises a computer for identification, a labeling machine for marking defective labels, an encoder, a front light source, a backlight source and an identification camera, the front light source is arranged above the backlight source, the front light source comprises a concentric-square-shaped light source table with a rectangular hole in the center, light sources ofthe light source table are evenly arranged on the two sides of the long edge of the rectangular hole. The identification camera is arranged above the rectangular hole; the identification camera, the front light source and the backlight source form a straight line vertical to the cloth; the output end of the identification camera is electrically connected with the computer for identification, the labeling machine is located behind the identification camera, the action end of the labeling machine is aligned with cloth, the control end of the labeling machine is connected with the computer for identification, the encoder is used for detecting the transmission speed of the current cloth, and the output end of the encoder is connected with the computer for identification.