Flexible expansion joint is relative to rigid expansion joint, and mechanical expansion joint belongs to rigid expansion joint. Its characteristics are that the expansion head of expansion joint is rigid, the periphery of expansion joint expands evenly, and the position of expansion joint must be cylindrical; Hydraulic expansion belongs to flexible expansion joint, including liquid bag expansion and rubber expansion. Its expansion head is flexible, characterized by uniform surrounding pressure, and elliptical holes can also achieve the effect of expansion joints.