To use a free warm piston instead of both an inertance tube and a buffer as the phase shifter of a Pulse Tube Cryocooler (PTC) not only can reduce the volume of the phase shifter, but also can improve the performance of the PTR. To study the influence of the free warm piston on the performance of the PTC, a free warm piston PTC is designed and studied experimentally. The results show that for a constant input power, when the working frequency increases, the displacement phase difference between the compressor piston and the free warm piston increases and the swept volume ratio of the compress piston over the free warm piston increases. When the charging pressure increases, the displacement phase difference increases and the swept volume ratio decreases. The results are of great significance to the design and optimization of free warm piston PTCs.