Aiming at the problem of low accuracy of straw coverage automatic recognition, it is proposed that a more accurate and adaptive method is used to detect straw coverage rate. Firstly, based on the color component spatial distance graying algorithm, the object and background of straw image collected by camera are separated; Secondly, the color image is grayed; Lastly, the straw image is binarized by improved Bernsen algorithm, and the straw coverage rate is calculated. In the experiment, 200 pictures of straw coverage are selected with coverage range of 20% ~ 30%, 30% ~ 40%, 40% ~ 50%, 50% ~ 60%, 60% ~ 70%, 70% ~ 80% and 80% ~ 90% respectively. The straw coverage rate is calculated respectively by the improved Bernsen algorithm and unimproved Bernsen algorithm. The result shows that the improved Bernsen algorithm is more accurate when the straw coverage is between 30% and 80%, and the error is less than 5%. In other cases, the calculation error of straw coverage rate is between 5% and 10%.