A streaming application upgrading method and a stream computing system, where the method includes obtaining a updated logical model of a streaming application, determining a to-be-adjusted stream by comparing the updated logical model with an initial logical model, generating an upgrading instruction according to the to-be-adjusted stream, and delivering the generated upgrading instruction to a worker node such that the worker node adjusts, according to an indication of the upgrading instruction, a stream between process elements (PEs) distributed on the worker node. The method provided in the present disclosure can upgrade the streaming application online without interrupting a service.