The invention relates to an automatic pipe pressure balancer with an axial expansion regulation function. The automatic pipe pressure balancer is characterized in that an upstream side pipe section and a downstream side pipe section form a nested flexible structure; the overlapped part of the two pipe sections form two isolating and sealing pressure action chambers in the water flow direction; the action area of a piston of each pressure action chamber is equal to that of the pipe downstream side. After the pressure of the upstream side pipe is led into the isolating and sealing pressure action chamber on the downstream side, an opposite balancing action force which is equal to water acting force applied to a model test device is generated on the piston in the isolating and sealing pressure action chamber on the downstream side so as to ensure that no water pressure acting force is applied to the model test device. Moreover, the piston can drive the downstream side pipe section to expand and contract in the the axial direction under the action of the pressure in the isolating and sealing pressure action chamber by injecting fluid with pressure into the isolating and sealing pressure chamber on one side.