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This study focused on investigating the effects of polypropylene fiber on the liquefaction resistance of saturated sand. We performed a battery of tests with a state-of-the-art ring shear apparatus on fiber-reinforced saturated sand, considering the influences of fiber content and sand density. Two different shearing methods named shear-torque-controlled (STC) and cyclic-torque-controlled (CTC) were considered for carrying out the tests. An energy approach was chosen to evaluate the results, and the fiber reinforcement mechanisms were analyzed. Our test results showed that in STC tests, the shear strength and shearing time of saturated sand increased proportionally to an increase of fiber content and sand density. The cycles required for liquefaction in CTC tests also increase with an increase in sand density and fiber content. The presence of fibers clearly increases the shear energy required for liquefaction. The shear energy increases with an increase in sand density and fiber content. Greater total shear energy is required in specimens with a higher density or larger fiber content. Fiber reinforcement in sand has acted as a spatial network in interlocking soil grains, thereby resulting in the necessity of more energy for overcoming the resistance during the shearing process. After performing the shearing test, the unreinforced specimen with loose structure collapsed totally, and the one with a dense structure collapsed partially, while fiber reinforcement specimens still maintained structural stability.