A network slice instance processing method and apparatus, and a system, the method including receiving, by a first network device, a first message sent by a second network device, where the first message requests that the first network device modify a first network slice subnet instance (NSSI) managed by the first network device, and where the first NSSI is shared by a plurality of network slice instances (NSIs) managed by different network devices, determining, by the first network device, whether the first NSSI is allowed to be modified, and performing, by the first network device, subsequent processing according to a determination of whether the first NSSI is allowed to be modified.