Although the fiber reinforced thermoplastic plastics (FRTP) materials has been widely applied in the automotive industry for non-load-bearing or semi-load-bearing parts, they are hardly used in load-bearing parts due to higher performance requirements. This paper focuses on an integrated design method for automotive load-bearing parts by using assembled continuous-FRTP laminates and non-continuous-FRTP materials (referred to as AFRTP). First, aiming for the lightest weight, an integrated material-structure-process design method is proposed. Next, taking a steel seat beam as a benchmark, the AFRTP beam is manufactured by using this method with two continuous steps: compression molding of the beam body and subsequent injection molding of the reinforcing rib. Finally, the feasibility of this method is verified though static axial compression, flexure, torsion and modal tests. Compared with the steel benchmark, the AFRTP beam weight is 32.4% lighter, the first order frequency of free mode is 6.5% higher, and flexural strength, axial compressive strength and peak torque increased by 58.2%, 13.2% and 64.8%, respectively. Through this study, the lightweight design is realized and feasibility of the integrated design method is verified. This work provides guidance for the practical lightweight designs of replacing metal with composite materials.