Abstract: For the large span and section flat, the stress and distortion of the construction and surrounding rock for large span and flat multiarch tunnel is differ from twolane multiarch tunnel or separated tunnel, optimal supporting time of secondary lining is influenced accordingly. Use the threelane multiarch tunnel for the supporting engineering, discuss the method use the displacement release ratio confirm the supporting time, and through site monitoring and synthesis analysis of surrounding rock displacement and secondary lining stress in difference supporting time, according to the principia of least supporting antistress, put forward the optimal supporting time of large span and flat multiarch tunnel.