The invention belongs to the technical field of coal mine supporting machines, and discloses a flexible supporting device for a steeply-inclined coal seam. The flexible supporting device comprises a left supporting plate, a left top plate, a right top plate, a right supporting plate, a left hydraulic cylinder, an upper hydraulic cylinder and a right hydraulic cylinder. The left supporting plate is hinged to the left top plate, the left top plate is hinged to the right top plate, the right top plate is hinged to the right supporting plate, the left supporting plate is hinged to one end of the left hydraulic cylinder, the other end of the left hydraulic cylinder is hinged to the left top plate, one end of the upper hydraulic cylinder is hinged to the left top plate, the other end of the upper hydraulic cylinder is hinged to the right top plate, one end of the right hydraulic cylinder is hinged to the right top plate 3, and the other end of the right hydraulic cylinder is hinged to the right supporting plate. According to the flexible supporting device, supporting force is stable, operation is easy and convenient, roof leakage of a supported mine tunnel does not occur easily, the supported mine tunnel is easy to transport, and safer and more reliable supporting and shielding are provided for coal mining under the condition of the steeply-inclined coal seam.