Abstract: Coarse cereals rich in polyphenol compounds (such as polyphenolic acids, tannin, anthocyanin, and phytosterols) can soften the blood vessels, and lower the blood sugar, blood lipids, and superoxide free radicals for anti-aging and the increasing physiological activity. In this study, a comparison was performed on the polyphenol content and antioxidant activity of seven coarse cereals grown in Northwest China. A systematic investigation was carried out to explore the effects of different processing on the antioxidant activity in vitro. The raw materials were taken as millet, yellow rice, oat, quinoa, brown rice, buckwheat, and sorghum rice. After that, the multigrain yoghurt was prepared with high antioxidant activity in vitro. The antioxidant activity of coarse cereals was evaluated by the Antioxidant potency composite (APC) index using three detection methods, namely, DPPH free radical scavenging capacity, ABTS+·scavenging capacity, and iron ion reducing capacity. The coarse cereal with better performance was processed using steaming, boiling, and beating. The polyphenol content and antioxidant activity were further compared. At the same time, the relationship was determined between the polyphenols and antioxidant activity of the coarse cereal under the best processing. Finally, the coarse cereal with better characteristics and processing was selected to produce the multigrain yoghurt with the optimal polyphenol content and antioxidant activity. The results showed that there were significant antioxidant activities of different coarse cereals (P<0.05). Specifically, brown rice and quinoa presented the strongest DPPH free radical scavenging capacity, followed by buckwheat. The buckwheat shared the strongest ABTS+·scavenging and iron reduction capacity. The antioxidant potency composite (APC) index was used to evaluate the antioxidant activity of different types of coarse cereals. The antioxidant activity was listed in the descending order of the buckwheat (93.45%) > the sorghum rice (52.42%) > the quinoa (51.24%) > the brown rice (50.91%) > the oats (39.70%) > the millet (33.82%) > the yellow rice (20.86%). The content of total phenols and flavonoids was also ranked in the descending order of steaming>boiling>beating. The contents of the total phenols were 329.48, 258.24, and 233.46 mg/100 g, respectively, whereas, that of the total flavonoids were 46.11, 34.89, and 34.09 mg/100g, respectively. According to the comprehensive antioxidant index, the antioxidant activity was ranked in the descending order of steaming (94.18%), boiling (91.52%), and beating (73.90%). The best antioxidant activity was achieved in the steamed buckwheat, indicating the lowest total phenol loss rate (29.22%), and the highest increase of flavonoids (34.75%). Furthermore, the correlation analysis between the polyphenol content and antioxidant activity of the steamed buckwheat showed that the total phenol, total flavonoid, and bound flavonoid presented a better correlation with the antioxidant activity. The total phenol showed an extremely significant positive correlation with the ABTS+·scavenging capacity and iron ion reducing capacity (P<0.001). A significant positive correlation was found between the bound flavonoid and DPPH radical scavenging capacity (P<0.01). The contents of free phenol, bound phenol, and total phenol in the buckwheat-yoghurt were significantly higher than those of the yoghurt without buckwheat (P<0.05). The total phenol content of buckwheat-yoghurt (52.85 mg/100g) was 5.16 times of that the yoghurt without buckwheat (10.94 mg/100g), whereas, the total flavonoid content of the buckwheat-yoghurt (12.49 mg/100g) was 4.23 times of that the yoghurt without buckwheat. This finding can provide a strong reference for coarse cereals and functional yoghurt.