In order to overcome the uncertainty of fault distribution in electrohydraulic servo system, an adaptive gain overtorsional sliding mode observer was designed to reconstruct the fault with bounded unknown disturbance in the electrohydraulic servo system. Through the analysis of the electrohydraulic servo system, the nonlinear dynamic equation of the electrohydraulic servo system was obtained. By using the nonlinear dynamic equation, an adaptive overtorsion observer was constructed to eliminate the conservativeness of the observer gain and improve the buffeting reduction effect. The adaptive overtorsion observer was introduced into the fault reconstruction of the electrohydraulic servo system to overcome the uncertainty of the fault distribution. The experimental results show that the designed method can be used to estimate the position and velocity of the cylinder better, the accuracy of the estimation results are high, there is no large bucket vibration phenomenon, and the overshoot is small. It shows that the designed adaptive overtorsion observer can better adapt to the uncertainties of faults and reduce the chattering effect, so as to better realize the faults reconstruction of the electrohydraulic servo system.