The invention relates to the technical field of rocket engines, particularly to a flexible extension effuser with self-adaption height, which includes a basic bell-shaped effuser part, a multi-stage bell-shaped extension part, a variable constrained point part and a support structure part, wherein part of the basic bell-shaped effuser part is a fixed effuser, one end of the fixed effuser is connected with a rocket engine, and one end is connected with the multi-grade bell-shaped extension part; the multi-grade bell-shaped extension part is a flexible extension effuser, is in a rolling state when in the launching initial state, and is unfolded along with the increase of flying height; the variable constrained point part is positioned on the flexible extension effuser, and is fixed to a setposition of the support structure part; the support structure part is positioned inside the flexible extension effuser, and is unfolded along with the increase of flying height. According to the flexible extension effuser, the current problem of low working efficiency of rockets with wide operation range, such as single-stage orbit rockets is solved, the air flow is kept fully expanded all the time, so that the rocket is in the optimal operation state at any height.