A system for online monitoring powder-based 3D printing processes and method thereof are disclosed. A uniform light source having a single wavelength provided by the system is irradiated onto the powder layer before and after applied with glue. Intensities of such reflected images are obtained and subtracted from each other in an image process procedure. A difference obtained through the subtraction is compared with an original 3D model in a computer. If any defect is found such as being larger than a threshold value, the powder-based 3D printing processes will be terminated. Therefore, the technical effects of online printing processes monitoring, time saving and printing resources saving will be achieved.