The dynamic link budget of low earth orbit (LEO) satellite data transmission link is analyzed, and the mathematical model of link efficiency is discussed. In order to enhance the data transmission link performance for exploration satellites, Variable Coding Modulation (VCM) system is applied to reduce link resource waste and improve the satellite-ground transmission throughput without increasing power consumption and design cost. A VCM schedule designing algorithm is illustrated for optimizing link throughput within the data transmission period. For a satellite system with foreseeable orbit and based on its dynamic link budget results, the proposed algorithm selects variable coding modulation combination under the bit error rate requirement to fit the link budget curve and keep a relatively constant link margin to achieve optimal link efficiency. Finally, on the basis of the proposed algorithm, a VCM data transmission system is designed for a LEO sun-synchronous satellite. According to the simulation results, the remarkable advantage of the proposed method is proved by comparison with traditional constant coding modulation design.