As for the long-time error propagation problem of the space station rendezvous phasing，a nonlinear covariance analysis method based on the unscented transformation (UTCAM) is presented. The strategy of rendezvous phasing and the computation method of orbit maneuver parameters is proposed，then the principles and flow of UTCAM are introduced. The comparison between UTCAM and the two methods of STK and the Monte-Carlo shows that the relative error of covariance analysis between those methods is under 1.2%, and the computation consumption time is only 1/460 that of Monte-Carlo, so it can accomplish the propagation of mean and covariance rapidly and exactly for nonlinear systems. Finally, the error propagation of a 20-day rendezvous phasing of space station is conducted by using UTCAM, and the results are validated by the Monte-Carlo simulation.