A fluid machine, heat exchanger, and operating method of fluid machine. The fluid machine includes: a rotation shaft (10), a cylinder (20), and a piston assembly (30). The rotation shaft (10) and the cylinder (20) are eccentrically disposed relative to each other and an eccentric distance is fixed. The piston assembly (30) has a variable volume chamber (31). Because the eccentric distance between the rotation shaft (10) and the cylinder (20) is fixed, the rotation shaft (10) and the cylinder (20) rotate about their respective axes thereof during motion and the position of center of mass remains unchanged, so that the piston assembly (30) is allowed to rotate stably and continuously when moving in the cylinder (20); and vibration of the fluid machine is mitigated, a regular pattern for changes in the volume of the variable volume cavity is ensured.