The work aims to solve the problem in manual reading of the hardness circle diameter in the hardness testing of the Certain ammunition body. A hardness testing device for the Certain ammunition body based on machine vision was designed by applying the material Brinell hardness testing principle, and a visual testing algorithm for the hardness of the Certain ammunition body was developed. According to the algorithm proposed, Maltlab was used to realize the extraction of the characteristics of the hardness circle of the Certain ammunition body and the calculation of the diameter. Finally, the results of the proposed testing method based on machine vision and the manual testing were compared and analyzed by T-test method, indicating that there was no difference between the two testing results. The designed testing device based on machine vision and the proposed visual testing algorithm have high reliability and accuracy, and can be applied to the hardness testing of the Certain ammunition body.