The large space wireless power transfer(WPT) system that receiving energy from the spatial magnetic field is an attractive alternative for low and medium power mobile devices. The bio-electromagnetic safety is the main issue of the large space WPT system. To reduce the intensity of the space magnetic field while meeting the received power demand, a novel design method based on the magnetic field in space is proposed for the large space WPT system. Different with the traditional magnetic coupling design method, the proposed method takes the spatial magnetic field as the design object, and the primary side and secondary side are designed separately. The relationships between the transferred power and efficiency of the receiver under certain spatial magnetic field are analyzed. The maximum transmission power of the receivers is introduced. The primary side is designed to satisfy the safety requirement, and the receiving side is designed to provide the request power and efficiency. Providing a guideline for the parameter design of the receivers. To verify the effectiveness of the proposed method, a 5 W, 6.78 MHz prototype with a 2-m diameter and 0.5-m distance double-circular single-turn coil is designed. The maximum transmission power state is verified by simulation. The simulated results show that by using the proposed method, the receiving side can satisfy the power requirements while the spatial magnetic field decreased.