Defoaming of foam drilling fluid on the ground is the key to its cyclic utilization. This paper proposes a novel mechanical foam breaker based on self-oscillation to promote the application of foam drilling technology. Simulation results showed that the combined effects of the negative pressure, collision, extrusion, and shear generated in the cavity cause the foam to fracture. A prototype was developed, and the effects of the structural parameters, components of the foam drilling fluid, cuttings, and temperature on the defoaming percentage were investigated in experiments. The experimental results showed that the diameter of the upper nozzle was the major structural factor influencing the defoaming. A smaller diameter for the upper nozzle resulted in a higher defoaming percentage. The defoaming percentage first increased and then decreased with increases in the lower nozzle diameter, cavity length, and diameter, which indicated that there is an optimum structure for the foam breaker. The cutting concentration and temperature could improve the defoaming percentage to some extent, and the salts had a minor influence on the defoaming percentage. The results verified the feasibility and advantages of using the mechanical foam breaker and laid a solid theoretical foundation for its application in petroleum and natural gas drilling. (C) 2018 Elsevier Ltd. All rights reserved.