Abstract: Single Gimbal Control Moment Gyro system is the best choose of large spacecraft attitude control system.But the phenomenon of singularity exists in this system.This increases difficulty in designing steering law.The singularity states of SGCMG system on the base of singularity analysis are researched,which have redundancy.The construction method of a new redundant SGCMG zero motion differential dynamic equation is established.The character of SGCMG system in the form of zero motion differential dynamic equation is given.The criterion of singularity escapable from reference 4 is improved.