The broomcorn millet (Panicum miliaceum L.) is widely cultivated in arid and semiarid areas due to better drought tolerance. The yield of different millet cultivars is closely related to photosynthesis and source-sink relationship. Knowledge is still scarce on the relationship between photosynthesis, source-sink relationship and yield in broomcorn millet. A field experiment was conducted to measure gas exchange parameters, chlorophyll fluorescence parameters, leaf area per plant, dry matter per plant, harvest index, yield and yield components to examine the response of yield to photosynthesis and source-sink relationship in nine broomcorn millet cultivars in semi-arid areas. The results showed that high light utilization ability (Pmax is higher) of millet showed obvious genetic variation, whereas low light utilization ability (α is lower) did not. Water use efficiency (WUE and iWUE) of different cultivars of millet showed significant positive correlation with yield, while net photosynthetic rate per leaf (Pn) had no such correlation with yield, which could be used as a target trait for millet cultivar selection. The genetic variation in source-sink relationship was significant in broomcorn millet, which enabled all the tested cultivars being divided into source-limited cultivars, sink-limited cultivars and cultivars with balanced source-sink relationship. Specifically, source-limited cultivars showed a lower leaf photosynthetic rate and leaf area per plant, resulting in a significant source limitation to yield (i.e. red broomcorn), whereas the photosynthetic potential was higher. Sink-limited cultivars had a higher photosynthesis, leaf area per plant and dry matter per plant, whereas lower harvest index (i.e. white broomcorn), leading to a significant sink limitation in yield for broomcorn millet. The cultivars in a good source-sink relationship had an intermediate photosynthetic rate, leaf area per plant and harvest index, finally resulting in the highest yield, such as Jingyuanzhongjiqing and Ningmei14. They also showed a higher transpiration rate and water use efficiency than other tested cultivars. This study will be beneficial to the breeding of broomcorn millet.