A fan blade device including: an upper hub member having an outer surrounding surface surrounding an axis extending in an axial direction (z); a lower hub member coupled to and disposed under said upper hub member, and having an external surrounding surface that surrounds the axis; a plurality of main fan blades extending outward from said outer surrounding surface of said upper hub member; and a plurality of extension fan blades extending outward from said external surrounding surface of said lower hub member and respectively abutting against said main fan blades; wherein: each main fan blade includes a main fan blade body that is arranged obliquely with respect to the axis, and at least one engaging groove that is recessed in a bottom surface of said main fan blade body; and each extension fan blade includes an extension fan blade body that is arranged obliquely with respect to the axis and that has a top surface being contiguously flush with the bottom surface of said main fan blade body of a respective one of said main fan blades, and at least one engaging piece that protrudes upward from said extension fan blade body and that engages said at least one engaging groove of a respective one of said main fan blades, characterised in that each main fan blade includes a plurality of first protrusions that protrude from said main fan blade body; and each extension fan blade includes a plurality of second protrusions that protrude from said extension fan blade body and that respectively abut against said first protrusions of the respective one of said main fan blades. an upper hub member having an outer surrounding surface surrounding an axis extending in an axial direction (z); a lower hub member coupled to and disposed under said upper hub member, and having an external surrounding surface that surrounds the axis; a plurality of main fan blades extending outward from said outer surrounding surface of said upper hub member; and a plurality of extension fan blades extending outward from said external surrounding surface of said lower hub member and respectively abutting against said main fan blades; wherein: each main fan blade includes a main fan blade body that is arranged obliquely with respect to the axis, and at least one engaging groove that is recessed in a bottom surface of said main fan blade body; and each extension fan blade includes an extension fan blade body that is arranged obliquely with respect to the axis and that has a top surface being contiguously flush with the bottom surface of said main fan blade body of a respective one of said main fan blades, and at least one engaging piece that protrudes upward from said extension fan blade body and that engages said at least one engaging groove of a respective one of said main fan blades, characterised in that each main fan blade includes a plurality of first protrusions that protrude from said main fan blade body; and each extension fan blade includes a plurality of second protrusions that protrude from said extension fan blade body and that respectively abut against said first protrusions of the respective one of said main fan blades. each main fan blade includes a main fan blade body that is arranged obliquely with respect to the axis, and at least one engaging groove that is recessed in a bottom surface of said main fan blade body; and each extension fan blade includes an extension fan blade body that is arranged obliquely with respect to the axis and that has a top surface being contiguously flush with the bottom surface of said main fan blade body of a respective one of said main fan blades, and at least one engaging piece that protrudes upward from said extension fan blade body and that engages said at least one engaging groove of a respective one of said main fan blades, characterised in that each main fan blade includes a plurality of first protrusions that protrude from said main fan blade body; and each extension fan blade includes a plurality of second protrusions that protrude from said extension fan blade body and that respectively abut against said first protrusions of the respective one of said main fan blades.