Work assistance is one of the important means to improve work efficiency. This paper focuses on substation work and proposes a real-time work assistance method based on petri net model. The work assistance includes two aspects, termed as, workflow guidance and operation prompt. Particularly, the workflow constructed by the petri net includes sequence, inhibition, concurrency and conflict structures to declare the correlations between work steps. Further, different forms of operation prompts at each step are designed, considering real-time responses of workers including skip, replay, pause and restart. The effectiveness of the method is well validated by an application of the work assistance on measuring the grounding current of transformer core and clamp. It is demonstrated that the unified assistance method built by the petri net successfully instructs standardized working process, helps avoid potential risks, and provides flexible operation prompts in real time.