Abstract: In fourth-generation reactors, fast reactors characterized by high average neutron flux, proliferating fission nuclear fuel, and transmutation of long-decay actinides have become the primary research focus in China. NECP-SARAX is an advanced reactor neutron analysis and calculation system independently developed by the Nuclear Engineering Computational Physics Laboratory of Xi'an Jiaotong University. Based on the features and functions of NECP-SARAX, Xi'an Jiaotong University has developed a customized LoongSARAX system specifically for core physical engineering design and safety evaluation of liquid metal cooled fast reactors. LoongSARAX includes cross-section generation module TULIP and core steady-state analysis module LAVENDER. TULIP is used for the calculation of uniform assemblies, and for addressing one-dimensional plate and cylinder problems. LAVENDER is used for the 3-D core calculation for advanced reactor neutron analysis systems. After obtaining the few-group cross sections for each assembly, LAVENDER employs multi-group neutron transport methods to calculate the core's primary parameters. LAVENDER calculates a variety of parameters for liquid metal cooled fast reactor cores, including criticality, power distribution, reactivity, burnup, etc. In the course of software development, discrepancies between the results obtained through software calculations and actual results are inevitable. If LoongSARAX is to be applied to engineering practice, it is indispensable to ensure its normative and systematic verification and validation (V&V). Relevant international data on the physical calculation benchmark of liquid metal cooled fast reactor were collected and collated in this paper, with a focus on the V&V of LoongSARAX. On this basis, the V&V matrix for the LoongSARAX program was established. In order to improve the accuracy of verification, the whole program was divided into different modules for module verification, subsystem verification and system verification respectively. LoongSARAX was referred to as the system, with TULIP and LAVENDER being its subsystems. The V&V process begins with modules in TULIP and LAVENDER, followed by the system as a whole. The V&V of LoongSARAX covers fast reactors with sodium and lead coolants, such as JOYO, ZPPR17A, FFTF, BFS, etc. By comparing the calculation results of LoongSARAX with the experimental measurement results or MCNP results in the benchmark reports, it can be clearly concluded that LoongSARAX has high calculation accuracy and reliability in the field of liquid metal cooled fast reactor. In summary, LoongSARAX, a computing system developed by Xi'an Jiaotong University for liquid metal cooled fast reactors, demonstrates excellent computing capabilities after undergoing rigorous and systematic V&V. This will provide strong support for the core physical engineering design and safety evaluation of liquid metal cooled fast reactors, and the application of LoongSARAX will also provide valuable experience for the development and promotion of fast reactors in the future.