[Purposes] To address the current situation where research on influence line identification primarily focuses on integral structures and lacks verification from actual bridge tests, a field measurement study is conducted on the influence line identification of a simply supported hollow slab girder bridge with a multi-girder system. [Methods] An actual bridge with a calculated span of 22m, composed of 16 laterally spliced main girders, is used as the test object. Utilizing the real-time kinematic (RTK) technology of a high-precision global navigation satellite system (GNSS) combined with photoelectric deflectometers, the spatial moving trajectory of the test vehicle and the dynamic response of the bridge are synchronously collected. By constructing a spatiotemporal mapping matrix, precise data alignment is achieved, and the deflection influence line of the bridge is inversely extracted based on the least squares method. [Findings] The experimental results demonstrate that the algorithm successfully inversely identifies smooth deflection influence lines with strictly convergent boundaries, aligning with the mechanical characteristics of simply supported girders. Field measurements indicate that the middle girders exhibit the maximum peak response, with the peak deflection influence coefficient reaching 0.02~0.025 mm/kN. This response attenuates significantly towards the side girders, where the peak coefficient drops to 0.0025~0.003 mm/kN, accurately reflecting the transverse load distribution mechanism among the main girders. [Conclusions] This scheme eliminates the need for complete traffic closure, providing a new economical, rapid, and reliable approach for assessing the safety status and quickly calculating the bearing capacity of bridges.