The characteristics and growth behavior of filamentation damage in K9 optical elements are researched by nanosecond laser with 1 064 nm and 355 nm. The results show that when the sample is irradiated with energy higher than the bulk damage threshold, the filamentous damage will be inevitably formed. The two kinds of laser irradiation will firstly form plasma channel in K9 optical elements, under subsequent laser irradiation, the damage sites near the incident laser absorb more laser energy, developing into a larger nucleation damage points, and gradually grow to the front surface, which is accompanied by the longitudinal expansion of a large number of cracks. The difference is that there are many filamentous channels when 355 nm laser induces filamentous damage, while 1 064 nm laser does not observe very obvious filamentous damage even when it is irradiated by laser energy far greater than the initial damage threshold. At the same time, it is found that the damage growth area of filamentous damage induced by two wavelengths increases exponentially with the increasing of laser irradiation times, and the damage growth rate caused by 355 nm is faster than that of 1 064 nm.