China’s dairy farming has presented a striking development in recent years. Under the dual constraints of environment and resources, it is of practical significance to increase the output of dairy farming and ensure the healthy and stable development of the dairy industry, by accurately comparing the differences in the farming efficiency of dairy farms at different scales and grasping key factors influencing the farming efficiency. This study, through the cost analysis of 263 scale farms across 23 provinces and regions of China in 2019, reaches a result that the cost of a single cow in a certain scale farm increases with the enlargement of the scale, and shows an inverted-U shaped curve with the relatively large scales (1 001 - 2 000 cows) at the highest point. It measures the farming efficiency of dairy farms at different scales through the data envelopment analysis and finds that the scale efficiency and allocation efficiency of scale farms in China are relatively high, while the technology efficiency and cost efficiency are relatively low. The efficiency of different scale farms is obviously different, where the cost efficiency, allocation efficiency and scale efficiency show a U-shaped curve as the scale enlarges (with the relatively large scale as the lowest point), while the technology efficiency gradually decreases as the scale expands. It is concluded that for the scale farms, feed conversion ratio and forage-to-concentrate ratio have a significantly negative impact on the scale efficiency, while the labor cost, number of employees, and depreciation of fixed assets are negatively correlated to the technology efficiency and cost efficiency of dairy farming.