The invention discloses a tunnel crushed zone construction method and a combined supporting device. Firstly, the arch-shaped tunnel wall is built, and after tunnel crushing is completed, concrete is injected by tightly attaching to the tunnel wall through an advanced small pipe; then a foundation is tamped, crushed soil below the tunnel wall is excavated, and foundation stabilization treatment isconducted below a tunnel; secondly, the supporting device is applied, a railway is laid on the foundation of the tunnel, all joints on a supporting arch support are fixed, and a tunnel supporting archsupport is erected on the railway; and finally, the arch supports are contracted, a new supporting plate is erected, the length of the railway is increased, concrete is continuously injected throughthe advanced small pipe, a stage stride supporting arch support is controlled to move on the railway, and thus supporting for the whole tunnel is conducted. The supporting device can move, thus supporting for the whole tunnel is completed, and the supporting device is of a combined structure, can be separated for supporting and can also facilitate maintenance for the later period; and additionally, a flexible adjustable mechanism is additionally arranged, and the situation of support collapse due to downward moving of the tunnel can be prevented.