Abstract: In order to promote the wide application of lightweight soil in geotechnical engineering, modified polypropylene fiber was added to improve its mechanical properties, the influencing factors of strength-deformation characteristics, compressive failure mode and unconfined compressive strength of fiber expanded polystyrene (EPS) particles lightweight soil were analyzed and discussed by unconfined compressive strength test, and the mechanics mechanism of fiber EPS particle lightweight soil was analyzed from micro level by SEM. The results show that when the EPS particle, fiber and cement contents are different, the stress-strain curves of fiber EPS particle lightweight soil are different. EPS and cement contents are the main factors which influence strength, and fiber content is the second. The strength decreases sharply with the increase of EPS content, and increases sharply with the increase of cement content. The EPS particle lightweight soil without adding fiber becomes loose and broken easily, and loses strength suddenly. The addition of fiber can enhance the peak strength, residual strength, integrity and toughness of lightweight soil, and improve its brittle failure mode. However, when the EPS content is higher (more than 3% for the mass of dry soil), the adhesion between fiber and cemented soil is limited, and the improvement performance for mechanical properties of EPS particle lightweight soil is weaker. EPS particle is hollow honeycomb structure, fiber is covered with the needle-like hydration of cement on the surfaces and forms space network structure. The conclusions obtained show that fiber improves the mechanical properties of lightweight soil.