A cushion device for a foldable chair includes a buffer member and a tension spring. The buffer member includes a sleeve portion for sleeving over a side part of a rear leg, a buffer portion provided on the sleeve portion and an embedded portion provided along a side of the sleeve portion and embedded inside a side channel of the rear leg and secured thereat via a fixing element. The tension spring includes a spring body having a first hooking portion fixed a pivot plate of a seat and a second hooking portion including a second extension section, a second bent section engaging a limiting plate of a front leg and a third extension section extending from the second bent section in such a manner to prevent disengagement of the second bent section of the tension spring from a hooking hole in the limiting plate.