With the rapid expansion of non-geostationary satellite orbit (NGSO) constellation systems, spectrum resources have become increasingly congested. To address the issue of spectrum sharing between NGSO and geostationary satellite orbit (GSO) systems, this paper proposes an interference-avoidance beam planning method for spaceborne phased array antennas, based on spatial isolation zones. By analyzing interference scenarios between NGSO and GSO systems, the Earth’s surface is discretized into a grid based on latitude and longitude for uniform spatial enumeration. A time-sliced aggregate interference analysis model is then established to assess the interference from NGSO systems to segments of the GSO arc. Based on the interference-to-noise ratio (I/N) threshold criterion, an optimization objective for phased array antenna beam planning is formulated to facilitate spectrum sharing between NGSO and GSO systems. Furthermore, the mapping mechanism between spatial isolation zones and beamforming strategies is elucidated, leading to the proposed method for interference-avoiding beam planning. Simulation results validate the effectiveness of the approach, offering a valuable reference for the design and implementation of low Earth orbit (LEO) satellite constellations.