A shunt active power filter (SAPF) is coupled with load admittance in weak grid conditions, which poses a challenge to stability analysis. In this article, the admittance model of the SAPF is developed, which reveals the coupling mechanism between SAPF and load concisely and accurately. On top of that, the stability region of the system is investigated. The passivity region of the load admittance to guarantee the system stability is first explored. However, the passivity region has a very narrowband at a specific frequency, which makes it difficult to achieve. The stability region is then studied and it turns out to be an extension of the passivity region, and especially the narrowband is expanded. Therefore, the stability region is much easier to meet. The stability-oriented design is then summarized in a flowchart. In the end, the effectiveness of the newly defined stability region is verified by experimental results.