To address the combined requirements for lightweight design and misalignment tolerance in wireless power transfer (WPT) systems for mobile load scenarios, this article proposes an engineering-oriented lightweight design method for a grid-type flat spiral pad (GFSP) magnetic coupler with a central windowed ferrite structure. First, GFSP coils are utilized as the transmitting and receiving units of the WPT system. Based on mutual inductance theory, a mechanism model is formulated to analyze the coupling coefficient under distributed winding, revealing how the winding configuration improves the coupling performance and reduces mutual inductance fluctuation under positional offsets. On this basis, a central opening is introduced in the ferrite plane of the GFSP structure. A corresponding parameter design process is proposed to balance structural weight reduction and coupling performance retention, and a representative parameter design is provided. Then, ANSYS/Maxwell simulation models of the original and lightweight GFSP magnetic couplers are established for comparative analysis of coupling coefficient retention. Results show that the proposed design significantly reduces ferrite weight while maintaining robust coupling performance under lateral misalignment, varying transmission distances, and vertical tilt conditions. Finally, a 200 W experimental prototype based on a single-switch dual-branch P#LCC-S compensation topology is established. Compared with the original design, the magnetic coupler̓s weight is reduced by 25%. Under 60% lateral offset, 90° vertical tilt, and 80~120 mm transmission distance, the system maintains an output voltage above 25 V and transfer efficiency above 80%, evaluating the effectiveness and engineering feasibility of the proposed design.