The invention provides a seat mounting frame and an assembly method thereof, and relates to the technical field of vehicles. The seat mounting frame comprises a first cross beam, a second cross beam,a first longitudinal beam and a second longitudinal beam, wherein the first longitudinal beam and the second longitudinal beam are arranged separately and used for being fixed to a vehicle body bottomplate; the first cross beam and the second cross beam are also arranged separately; both of the first cross beam and the second cross beam are bonded to the upper end surface of the first longitudinal beam at one end and to the upper end surface of the second longitudinal beam at the other end; and the first cross beam, the second cross beam, the first longitudinal beam and the second longitudinal beam are all aluminum alloy profiles. The seat mounting frame relieves the problem of high cost of molds because parts of the seat mounting frame are formed by sheet metal stamping in the prior art.