Abstract: As a key northern branch of the Tanlu tectonic zone, the Jiamusi-Yitong fault zone has influenced basin development, magmatic activity, and earthquake distribution in Northeast China and exhibits a complex, segmented structure. However, the Luobei segment at its northern end is poorly understood because of intense tectonic overprinting and a lack of research data, particularly the lack of high-precision structural attributes. To address this gap, in this study, first-arrival P-wave tomography was conducted using a newly acquired deep seismic reflection profile to obtain the high-resolution P-wave velocity structure of the upper crust in the fault zone. During tomographic inversion, the finite element method was used for mesh generation to account for the substantial topographic relief and complex basement morphology in the study area. In addition, a single-channel picking error control parameter was introduced to address variations in data quality and ensure inversion stability. The results show that the Luobei segment of the Jiamusi-Yitong fault zone is composed of multiple branch faults, leading to clear lateral differences in velocity at the top of the crust in this area. Based on the distribution characteristics of the medium- and low-velocity anomalies along the profile, four branch faults with different movement and activity characteristics were identified, among which the easternmost branch fault had the strongest activity and extensional characteristics. This study advances high-precision seismic detection techniques and accurately depicts the structural framework of the upper part of the fault zone. It provides velocity constraints that enhance our understanding of regional tectonic evolution and improve existing geophysical models.