Traditional contact strain gauges present limitations in applications such as aerospace structure testing, including susceptibility to introducing interference and lacking full-field measurement capability. To overcome these challenges, this paper studies and implements a non-contact, full-field strain measurement method based on digital speckle image technology. A stereo vision system was used to collect images of an equal-strength beam specimen with a random speckle pattern prepared on its surface, under loads ranging from -20 N to +20 N. By employing a digital image correlation algorithm to calculate the displacement field of the specimen surface, the average axial strain in a specified area was subsequently obtained. The digital speckle image measurement results were systematically compared and analyzed with the measurement results from a resistance strain gauge system (full-bridge configuration) attached to the same area of the same specimen, as well as theoretical calculated values. Results show that within the range of ±176 με, the average measurement error of the digital speckle image measurement system was reduced by 5.3% compared to the strain gauge measurement system. Furthermore, through linear fitting analysis of the measurement data, the digital speckle image measurement system exhibited certain advantages in terms of sensitivity and zero stability; its strain-vs-load curve was closer to an ideal linear relationship and showed no zero offset.