Aiming at the disadvantages of the excisting methods of indicated torque estimation,a new method is brought out based on instantaneous speed according to the kinetic model. A new concept is bring into the method brought out in this paper——synthetic acceleration, to eliminate the effect of the non-linear part in the model. Regarding the synthetic acceleration as input and the indicated torque as output, the armax model is used to build the transfunction, so the indicated torque estimation based on instantaneous speed is realized.