The invention relates to a Kalman filtering based online calibrating method of a Stewart mechanism. The calibrating method comprises the steps that 1) a pose measuring device is fixed at one side of the Stewart mechanism, and the working space of the pose measuring device covers the motion range of the Stewart mechanism; 2) the pose of the Stewart mechanism is adjusted so that a moving platform of the Stewart mechanism moves in different directions, the pose measuring device measures the pose of the moving platform of the Stewart mechanism, and measured pose data is compared with theoretical pose data to obtain pose error data; 3) the theoretical pose data and the pose error data are substituted into the Kalman filtering algorithm, and error source data is obtained via iteration; 4) the error source data is substituted into the kinematic positive solution of the Stewart mechanism to correct the kinematic positive solution; and 5) the steps 2) to 4) are implemented cycled till the pose error data is converged. According to the invention, operation is simple, the cost is low, and the calibration efficiency is high.