Abstract: To clarify the mass ratio effect on the flow-induced vibration of two tandem square cylinders, numerical simulation is employed to investigate the effect of mass ratio (m^\ast =3, 10, 20) on the vibration response characteristic of the downstream square cylinder at Re=150. The evolution of the wake modes are discussed, and the fluid-structure interaction (FSI) mechanism of the downstream square cylinder is analyzed as well. The results show that the mass ratio plays an important role in the flow-induced vibration of the downstream cylinder. When the mass ratio is small (m^\ast =3), the vibration response is very complicated for the downstream cylinder. With the increase of reduced flow speed, the downstream cylinder does not have the traditional lock-in phenomenon (with the lock-in frequency ratio around 1) but has the soft-lock-in phenomenon (with the lock-in frequency ratio less than 1). When the mass ratio is large (m^\ast =10 and 20), the soft-lock-in phenomenon disappears, while the traditional lock-in phenomenon occurs instead. The maximum transverse amplitude of the downstream cylinder decreases gradually with the increase of the mass ratio. Furthermore, the mass ratio has an obvious effect on the distance between two square cylinders. The distance between two cylinders severely decreases in the lock-in region for the small mass ratio but keeps almost constant for the larger mass ratio. In addition, the mass ratio also has a significant effect on the wake modes and FSI mechanisms of two tandem square cylinders. For the small mass ratio (m^\ast =3), the wake modes and the FSI mechanism are very diverse.