This work aimed to study the degradation property of the Nano-cellulose crystal/poly (lactic acid) composite under four different degradation conditions. Degradation experiments were conducted with homemade NCC/PLA composite films in different solutions (pH=3, pH=7 and pH=11) and under ultraviolet, then the relationship between the degradation property of composite films (before and after degradation) and pH, light, NCC was analyzed through the weight loss rate measurement, scanning electron microscopy and X-ray photoelectron spectroscopy. Furthermore, its degradation mechanism was studied, and compared with that of pure PLA films. The degradation rate of NCC/PLA composite films in alkaline condition was the fastest, followed by acidic condition, and then neutral condition, while the degradation in UV irradiation condition was the slowest. The degradation rate of NCC/PLA composite film was faster than that of pure PLA film under different degradation conditions. O/C of NCC /PLA composite film increased after the acid, alkali, ultraviolet degradation by 35.16%, 36.66% and 38.65% respectively as compared to that before degradation. Addition of nano-cellulose crystal improved the degradation property of the NCC/PLA composite film. In different degradation processes, the ratio of carbon atoms on the composite films decreased, and correspondingly the ratio of Oxygen atoms increased, therefore the O/C ratio increased.