The invention discloses a fiber-reinforced square tube section bar compound extruding device and an extruding method thereof. The compound extruding device comprises a compound extruding mould and a fiber traction device, wherein the compound extruding mould comprises a guide plate, an upper mould, a lower mould, a first mandrel, a second mandrel, feeding plates and a mould core; the upper mould is connected with the first mandrel, the mould core and the second mandrel; a through hole is formed in each of the first mandrel and the second mandrel; two pairs of blocking slots are formed betweenthe guide plate and the upper mould; the feeding plates are blocked in the blocking slots; fiber feeding channels are arranged on the feeding plates; and a welding-on chamber, a mould hole and an airknife which communicate with one another in sequence are arranged on the lower mould. According to the compound extruding device and the extruding method thereof disclosed by the invention, section bars can be continuously reinforced on specific positions of cross sections of the section bars, so that continuous and compound extrusion is realized, and infinitely-long fiber-reinforced section barsare manufactured; and meanwhile, compared with conventional fiber particle mixed blanks, a conventional blank is used, so that production cost is reduced, and wear resistance of the mould is also improved.