The invention relates to an online monitoring device of a laser spatial filter and a spatial filter. The online monitoring device comprises a total reflection prism, a lens, a light source and an observation unit, wherein the total reflection prism is provided with a first transmission surface and a second transmission surface which are vertical to each other and a transmission surface, the lens is parallel to the first transmission surface of the total reflection prism, and the light source is vertically irradiated to the first transmission surface. After the online monitoring device is adopted, targeted observation points in a narrow small space can be monitored, so the problem that a charge coupled device (CCD) photographic device cannot be arranged in the laser transmission forward direction of the traditional high power laser device to observe the laser transmission state is solved.