The suitable ecological water level is a key element to maintain the stability of lake ecosystems, as well as an important indicator reflecting the hydrological situation of lakes. Most studies of suitable ecological water level focused on the hydrological processes. There was lack of research on all-season ecological water level thresholds coupled with lake nutrient loads. On the basis of depicting the restored natural water level, a comprehensive framework for the suitable ecological water level calculation was built with consideration of lake nutrient load caused by human activities and minimum ecological water level during ice-covered period. The suitable ecological water level thresholds at different periods were determined in our study. The results showed that both total nitrogen (TN) concentration and water level of the rivers entering the lake in the non-freezing period of Lake Chagan had an upward trend; the total phosphorus(TP) concentration showed a downward trend; the maximum retention of total nitrogen and total phosphorus in the lake reached 1044.36 t and 23.61 t in June; the suitable ecological water level was 129 m in the ice-covered period (from November to next year April), 130.15-130.86 m in the flood season (from June to September), and 130.08-130.57 m in the non-flood season (May and October) respectively. The results can provide method support for the study of the suitable ecological water level of lakes with seasonal ice cover, and provide theoretical support for the water environment management of lakes.