Objective To assess the injury risks and affecting factors of spinal loads at L5/S1 joint for caregivers during manual patient-handling tasks, so as to find a suitable handling way to effectively reduce such risks. Methods Kinetic data were collected from nine female caregivers performing six patient handling tasks on three agent ‘patients’ with different weight. Peak L5/S1 force was calculated based on a multi-segment three-dimensional model to investigate the characteristics of injury risks, and the correlations between the peak L5/S1 force and six affecting factors were specifically analyzed. Results Peak compression of the tasks all exceeded the safe thresholds of 3.4 kN; Peak anterior-posterior force for those repositioning tasks were above the threshold of 0.5 kN. Different tasks showed different biomechanical characteristics for risk of injury and affecting factors, and there was a significant correlation between the L5/S1 loads and the moving distance of center of gravity, and the ground reaction force as well (P<0.01). Conclusions Manual patient handling tasks could cause different injury risks by high loads on spinal low back. Decreasing the range of motion of the whole body and avoiding jerk movement during the patient handling tasks would help caregiver to reduce the injury risks.