The invention discloses a spaceborne GPS orbit determination method based on adaptive measurement noise variance estimation. The method comprises steps: (1) according to I and Q paths of signals in a spaceborne GPS receiver tracking loop, a narrowband broadband power ratio method is used to estimate a carrier noise ratio (C/N0) of the signals; (2) according to the C/N0 of a tracking loop in each channel, pseudo distance measurement errors between L1 and L2 dual-frequency signals are calculated; (3) L1 and L2 dual-frequency pseudo distance measurement values are combined to obtain a pseudo distance measurement value rhoIF with no ionosphere delay and the error sigmaIF; and (4) with the rhoIF as an observed quantity, an EKF algorithm is used to estimate the orbit information of the satellite. In the method, the measurement noise variance matrix is adaptively adjusted through monitoring changes of the C/N0 of signals in each channel of the spaceborne GPS receiver in real time, the noise statistical characteristics of the observed quantity can be reflected correctly, defects existing in insufficient priori information and dynamic environment of the traditional fixed design EKF algorithm are overcome, and a higher orbit determination precision can be acquired.