The invention discloses an online dynamic calibrating method for errors of inner and outer lever arms of two rotary inertial navigation systems. The method comprises the steps: firstly, establishing measuring models for respective inner lever arms, a measuring model for intersystem outer lever arm errors and a measuring model for intersystem speed difference of the two rotary inertial navigation systems; secondly, realizing a principle and way of separation of the respective inner lever arms of the two rotary inertial navigation systems on the basis of model analysis, and designing a rotationpolicy of the two rotary inertial navigation systems; then, separately controlling frameworks of RINS1 and RINS2 to rotate according to the preset rotation policy, carrying out navigation resolving, and outputting speed information on the two rotary inertial navigation systems; finally, calculating speed difference between the two rotary inertial navigation systems, constructing a Kalman filter bytaking the speed difference as a measurement amount, so as to achieve the online dynamic calibration on the errors of the inner and outer lever arms of the two rotary inertial navigation systems. According to the method, the required calibration time is short, the calibration process is simple, the accuracy in calibration of the inner and outer lever arms is high, the dependence on other outsidereference information is not required, the independence is high, and the implementation is facilitated.