The coupling dynamics of the pitching of spacecraft, thesloshing of liquid fuel and the vibration of elastic appendages areinvestigated. The coupling dynamics equations deduced by using H-O principleare adopted. The attitude of spacecraft is controlled. The multiple scalemethod is proposed to analyze the 1:1:1 internal resonance of therigid-liquid-elastic coupling system. The effect mechanism of the couplingdynamics are obtained by the analytic study. The amplitude-frequencyresponse switches between soft and hard spring type with liquid depth. Theresonances of the spacecraft and elastic appendages occur at both of thenatural frequencies of spacecraft and elastic appendages.