Leaf nitrogen content (LNC) can be used to diagnose the nutritional status and guide precise fertilization. Numerous previous researchers estimated LNC on reflectance spectrum or active chlorophyll fluorescence techniques with certain limitations. This study proposed a new technique of sun-induced chlorophyll fluorescence (SIF) for detecting LNC. We conducted an experiment to determine the optimal measurement point at the leaf scale for SIF and the best fluorescence yield indices (FY indices) extracted from SIF for LNC detection. The differences of the upward and downward FY indices were compared to determine the optimal FY indices. The results showed that the 1/3 distance from the leaf base is the optimal position for measuring SIF at the leaf level, and the downward FY indices are much better than the upward one with ratio peak FY687/FY739 as the best to monitor the LNC. The findings demonstrated that SIF can be utilized as a potential method for monitoring the LNC of winter wheat with higher efficiency.