Tractor-mounted harvesters are operated under harsh conditions to dig deep-rooted crops on irregular soil conditions. Therefore, it is important to evaluate their fatigue strength by measuring random load characteristics that occur during operation. Based on this, highly reliable fatigue design that can be used safely without damage during the service life is required. In this study, the traction force of a tractor-mounted garlic-onion harvester was measured through a field test, and its fatigue damage was calculated based on the Rain-flow counting method and Goodman’s equation. In addition, a method of verifying the fatigue safety of the harvester by calculating the equivalent load and required test cycle for the accelerated life test in the laboratory was presented. The average traction force calculated from the torque history measured in field tests by attaching a torque meter to four tractor wheels at the actual harvesting site was found to be 8.51 kN. When the accelerated life test condition corresponding to the target life of 800 h was calculated using the presented method, the number of load cycles required under a maximum load of 12.37 kN and a minimum load of 7.56 kN was found to be 2.67 × 106 cycles.